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Wednesday, May 30, 2007

Dotnet Tutorials

Here are 29 books, tutorials, and chapters for learning about developing applications using the .Net (DotNET) Framework using C# (C Sharp), Visual Basic .NET, and ASP .NET. These learning and training materials are from many different web resources, and give you a great start or refresher in these Microsoft development technologies. All of these are in pdf format.

DotNET Progamming Books and Tutorials
An Overview of the .NET Technology
DotNET Tutorial
J2EE vs. Microsoft .NET
Sample Applications for .NET developers
Eiffel .Net (DotNet-XML-tutorial)
Security in the Microsoft .NET Framework

C# Progamming Books and Tutorials
C# Language Specification
A Programmer’s Introduction to C#, Second Edition
C# for Programmers
Basic Introduction to C#
C SharpThreading, Part I Intermediate Level Creating Threads
C Sharp Threading, Part II Intermediate Level
What is C# and Basic Requirements for C#
Introduction to C#
Advanced C#
Detailed C# Language Specification
Sharpen Up on C#
Visual C# .NET Developer's Handbook
User Interfaces in C#: Windows Forms and Custom Controls
Chapter 5: Advanced Class Design
Active C#

Visual Basic .Net Progamming Books and Tutorials
Visual Basic .NET Essentials For Experienced Programmers
Build Your Own ASP .NET Website Using C# & VB .NET
VB .NET coding standard

Migrating from COM and VB6 to VB .NET

ASP .NET Progamming Books and Tutorials
Introduction to ASP .NET
ASP .NET Introduction
An Introduction to ASP .NET
ASP .NET Fundamentals Introduction to ASP .NET 2.0 (VB .NET)
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Dotnet faqs

1. Introduction

1.1 What is .NET?

.NET is a general-purpose software development platform, similar to Java. At its core is a virtual machine that turns intermediate language (IL) into machine code. High-level language compilers for C#, VB.NET and C++ are provided to turn source code into IL. C# is a new programming language, very similar to Java. An extensive class library is included, featuring all the functionality one might expect from a contempory development platform - windows GUI development (Windows Forms), database access (ADO.NET), web development (ASP.NET), web services, XML etc.

See also Microsoft's definition.

1.2 When was .NET announced?

Bill Gates delivered a keynote at Forum 2000, held June 22, 2000, outlining the .NET 'vision'. The July 2000 PDC had a number of sessions on .NET technology, and delegates were given CDs containing a pre-release version of the .NET framework/SDK and Visual Studio.NET.

1.3 What versions of .NET are there?

The final versions of the 1.0 SDK and runtime were made publicly available around 6pm PST on 15-Jan-2002. At the same time, the final version of Visual Studio.NET was made available to MSDN subscribers.

.NET 1.1 was released in April 2003, and was mostly bug fixes for 1.0.

.NET 2.0 was released to MSDN subscribers in late October 2005, and was officially launched in early November.

1.4 What operating systems does the .NET Framework run on?

The runtime supports Windows Server 2003, Windows XP, Windows 2000, NT4 SP6a and Windows ME/98. Windows 95 is not supported. Some parts of the framework do not work on all platforms - for example, ASP.NET is only supported on XP and Windows 2000/2003. Windows 98/ME cannot be used for development.

IIS is not supported on Windows XP Home Edition, and so cannot be used to host ASP.NET. However, the ASP.NET Web Matrix web server does run on XP Home.

The .NET Compact Framework is a version of the .NET Framework for mobile devices, running Windows CE or Windows Mobile.

The Mono project has a version of the .NET Framework that runs on Linux.

1.5 What tools can I use to develop .NET applications?

There are a number of tools, described here in ascending order of cost:

You can see the differences between the various Visual Studio versions here.

1.6 Why did they call it .NET?

I don't know what they were thinking. They certainly weren't thinking of people using search tools. It's meaningless marketing nonsense.

2. Terminology

2.1 What is the CLI? Is it the same as the CLR?

The CLI (Common Language Infrastructure) is the definiton of the fundamentals of the .NET framework - the Common Type System (CTS), metadata, the Virtual Execution Environment (VES) and its use of intermediate language (IL), and the support of multiple programming languages via the Common Language Specification (CLS). The CLI is documented through ECMA - see http://msdn.microsoft.com/net/ecma/ for more details.

The CLR (Common Language Runtime) is Microsoft's primary implementation of the CLI. Microsoft also have a shared source implementation known as ROTOR, for educational purposes, as well as the .NET Compact Framework for mobile devices. Non-Microsoft CLI implementations include Mono and DotGNU Portable.NET.

2.2 What is IL?

IL = Intermediate Language. Also known as MSIL (Microsoft Intermediate Language) or CIL (Common Intermediate Language). All .NET source code (of any language) is compiled to IL during development. The IL is then converted to machine code at the point where the software is installed, or (more commonly) at run-time by a Just-In-Time (JIT) compiler.

2.3 What is C#?

C# is a new language designed by Microsoft to work with the .NET framework. In their "Introduction to C#" whitepaper, Microsoft describe C# as follows:

"C# is a simple, modern, object oriented, and type-safe programming language derived from C and C++. C# (pronounced “C sharp”) is firmly planted in the C and C++ family tree of languages, and will immediately be familiar to C and C++ programmers. C# aims to combine the high productivity of Visual Basic and the raw power of C++."

Substitute 'Java' for 'C#' in the quote above, and you'll see that the statement still works pretty well :-).

If you are a C++ programmer, you might like to check out my C# FAQ.

2.4 What does 'managed' mean in the .NET context?

The term 'managed' is the cause of much confusion. It is used in various places within .NET, meaning slightly different things.

Managed code: The .NET framework provides several core run-time services to the programs that run within it - for example exception handling and security. For these services to work, the code must provide a minimum level of information to the runtime. Such code is called managed code.

Managed data: This is data that is allocated and freed by the .NET runtime's garbage collector.

Managed classes: This is usually referred to in the context of Managed Extensions (ME) for C++. When using ME C++, a class can be marked with the __gc keyword. As the name suggests, this means that the memory for instances of the class is managed by the garbage collector, but it also means more than that. The class becomes a fully paid-up member of the .NET community with the benefits and restrictions that brings. An example of a benefit is proper interop with classes written in other languages - for example, a managed C++ class can inherit from a VB class. An example of a restriction is that a managed class can only inherit from one base class.

3. Assemblies

3.1 What is an assembly?

An assembly is sometimes described as a logical .EXE or .DLL, and can be an application (with a main entry point) or a library. An assembly consists of one or more files (dlls, exes, html files etc), and represents a group of resources, type definitions, and implementations of those types. An assembly may also contain references to other assemblies. These resources, types and references are described in a block of data called a manifest. The manifest is part of the assembly, thus making the assembly self-describing.

An important aspect of assemblies is that they are part of the identity of a type. The identity of a type is the assembly that houses it combined with the type name. This means, for example, that if assembly A exports a type called T, and assembly B exports a type called T, the .NET runtime sees these as two completely different types. Furthermore, don't get confused between assemblies and namespaces - namespaces are merely a hierarchical way of organising type names. To the runtime, type names are type names, regardless of whether namespaces are used to organise the names. It's the assembly plus the typename (regardless of whether the type name belongs to a namespace) that uniquely indentifies a type to the runtime.

Assemblies are also important in .NET with respect to security - many of the security restrictions are enforced at the assembly boundary.

Finally, assemblies are the unit of versioning in .NET - more on this below.

3.2 How can I produce an assembly?

The simplest way to produce an assembly is directly from a .NET compiler. For example, the following C# program:

    public class CTest
   {
       public CTest() { System.Console.WriteLine( "Hello from CTest" ); }
   }

can be compiled into a library assembly (dll) like this:

    csc /t:library ctest.cs

You can then view the contents of the assembly by running the "IL Disassembler" tool that comes with the .NET SDK.

Alternatively you can compile your source into modules, and then combine the modules into an assembly using the assembly linker (al.exe). For the C# compiler, the /target:module switch is used to generate a module instead of an assembly.

3.3 What is the difference between a private assembly and a shared assembly?

The terms 'private' and 'shared' refer to how an assembly is deployed, not any intrinsic attributes of the assembly.

A private assembly is normally used by a single application, and is stored in the application's directory, or a sub-directory beneath. A shared assembly is intended to be used by multiple applications, and is normally stored in the global assembly cache (GAC), which is a central repository for assemblies. (A shared assembly can also be stored outside the GAC, in which case each application must be pointed to its location via a codebase entry in the application's configuration file.) The main advantage of deploying assemblies to the GAC is that the GAC can support multiple versions of the same assembly side-by-side.

Assemblies deployed to the GAC must be strong-named. Outside the GAC, strong-naming is optional.

3.4 How do assemblies find each other?

By searching directory paths. There are several factors that can affect the path (such as the AppDomain host, and application configuration files), but for weakly named assemblies the search path is normally the application's directory and its sub-directories. For strongly named assemblies, the search path is the GAC followed by the private assembly path.

3.5 How does assembly versioning work?

An assembly has a version number consisting of four parts, e.g. 1.0.350.1. These are typically interpreted as Major.Minor.Build.Revision, but this is just a convention.

The CLR applies no version constraints on weakly named assemblies, so the assembly version has no real significance.

For strongly named assemblies, the version of a referenced assembly is stored in the referring assembly, and by default only this exact version will be loaded at run-time. If the exact version is not available, the referring assembly will fail to load. It is possible to override this behaviour in the config file for the referring assembly - references to a single version or a range of versions of the referenced assembly can be redirected to a specific version. For example, versions 1.0.0.0 to 2.0.0.0 can be redirected to version 3.0.125.3. However note that there is no way to specify a range of versions to be redirected to. Publisher policy files offer an alternative mechanism for redirecting to a different version for assemblies deployed to the GAC - a publisher policy file allows the publisher of the assembly to redirect all applications to a new version of an assembly in one operation, rather than having to modify all of the application configuration files.

The restrictions on version policy for strongly named assemblies can cause problems when providing patches or 'hot fixes' for individual assemblies within an application. To avoid having to deploy config file changes or publisher policy files along with the hot fix, it makes sense to reuse the same assembly version for the hot fix. If desired, the assemblies can be distinguised by altering the assembly file version, which is not used at all by the CLR for applying version policy. For more discussion, see Suzanne Cook's When to Change File/Assembly Versions blog entry.

Note that the versioning of strongly named assemblies applies whether the assemblies are deployed privately or to the GAC.

3.6 How can I develop an application that automatically updates itself from the web?

For .NET 1.x, use the Updater Application Block. For .NET 2.x, use ClickOnce.

4. Application Domains

4.1 What is an application domain?

An AppDomain can be thought of as a lightweight process. Multiple AppDomains can exist inside a Win32 process. The primary purpose of the AppDomain is to isolate applications from each other, and so it is particularly useful in hosting scenarios such as ASP.NET. An AppDomain can be destroyed by the host without affecting other AppDomains in the process.

Win32 processes provide isolation by having distinct memory address spaces. This is effective, but expensive. The .NET runtime enforces AppDomain isolation by keeping control over the use of memory - all memory in the AppDomain is managed by the .NET runtime, so the runtime can ensure that AppDomains do not access each other's memory.

One non-obvious use of AppDomains is for unloading types. Currently the only way to unload a .NET type is to destroy the AppDomain it is loaded into. This is particularly useful if you create and destroy types on-the-fly via reflection.

Microsoft have an AppDomain FAQ.

4.2 How does an AppDomain get created?

AppDomains are usually created by hosts. Examples of hosts are the Windows Shell, ASP.NET and IE. When you run a .NET application from the command-line, the host is the Shell. The Shell creates a new AppDomain for every application.

AppDomains can also be explicitly created by .NET applications. Here is a C# sample which creates an AppDomain, creates an instance of an object inside it, and then executes one of the object's methods:

    using System;
   using System.Runtime.Remoting;
   using System.Reflection;

   public class CAppDomainInfo : MarshalByRefObject
   {
       public string GetName() { return AppDomain.CurrentDomain.FriendlyName; }
   }

   public class App
   {
       public static int Main()
       {
           AppDomain ad = AppDomain.CreateDomain( "Andy's new domain" );
           CAppDomainInfo adInfo = (CAppDomainInfo)ad.CreateInstanceAndUnwrap(
               Assembly.GetCallingAssembly().GetName().Name, "CAppDomainInfo" );       
           Console.WriteLine( "Created AppDomain name = " + adInfo.GetName() );
           return 0;
       }
   }

4.3 Can I write my own .NET host?

Yes. For an example of how to do this, take a look at the source for the dm.net moniker developed by Jason Whittington and Don Box. There is also a code sample in the .NET SDK called CorHost.

5. Garbage Collection

5.1 What is garbage collection?

Garbage collection is a heap-management strategy where a run-time component takes responsibility for managing the lifetime of the memory used by objects. This concept is not new to .NET - Java and many other languages/runtimes have used garbage collection for some time.

5.2 Is it true that objects don't always get destroyed immediately when the last reference goes away?

Yes. The garbage collector offers no guarantees about the time when an object will be destroyed and its memory reclaimed.

There was an interesting thread on the DOTNET list, started by Chris Sells, about the implications of non-deterministic destruction of objects in C#. In October 2000, Microsoft's Brian Harry posted a lengthy analysis of the problem. Chris Sells' response to Brian's posting is here.

5.3 Why doesn't the .NET runtime offer deterministic destruction?

Because of the garbage collection algorithm. The .NET garbage collector works by periodically running through a list of all the objects that are currently being referenced by an application. All the objects that it doesn't find during this search are ready to be destroyed and the memory reclaimed. The implication of this algorithm is that the runtime doesn't get notified immediately when the final reference on an object goes away - it only finds out during the next 'sweep' of the heap.

Futhermore, this type of algorithm works best by performing the garbage collection sweep as rarely as possible. Normally heap exhaustion is the trigger for a collection sweep.

5.4 Is the lack of deterministic destruction in .NET a problem?

It's certainly an issue that affects component design. If you have objects that maintain expensive or scarce resources (e.g. database locks), you need to provide some way to tell the object to release the resource when it is done. Microsoft recommend that you provide a method called Dispose() for this purpose. However, this causes problems for distributed objects - in a distributed system who calls the Dispose() method? Some form of reference-counting or ownership-management mechanism is needed to handle distributed objects - unfortunately the runtime offers no help with this.

5.5 Should I implement Finalize on my class? Should I implement IDisposable?

This issue is a little more complex than it first appears. There are really two categories of class that require deterministic destruction - the first category manipulate unmanaged types directly, whereas the second category manipulate managed types that require deterministic destruction. An example of the first category is a class with an IntPtr member representing an OS file handle. An example of the second category is a class with a System.IO.FileStream member.

For the first category, it makes sense to implement IDisposable and override Finalize. This allows the object user to 'do the right thing' by calling Dispose, but also provides a fallback of freeing the unmanaged resource in the Finalizer, should the calling code fail in its duty. However this logic does not apply to the second category of class, with only managed resources. In this case implementing Finalize is pointless, as managed member objects cannot be accessed in the Finalizer. This is because there is no guarantee about the ordering of Finalizer execution. So only the Dispose method should be implemented. (If you think about it, it doesn't really make sense to call Dispose on member objects from a Finalizer anyway, as the member object's Finalizer will do the required cleanup.)

For classes that need to implement IDisposable and override Finalize, see Microsoft's documented pattern.

Note that some developers argue that implementing a Finalizer is always a bad idea, as it hides a bug in your code (i.e. the lack of a Dispose call). A less radical approach is to implement Finalize but include a Debug.Assert at the start, thus signalling the problem in developer builds but allowing the cleanup to occur in release builds.

5.6 Do I have any control over the garbage collection algorithm?

A little. For example the System.GC class exposes a Collect method, which forces the garbage collector to collect all unreferenced objects immediately.

Also there is a gcConcurrent setting that can be specified via the application configuration file. This specifies whether or not the garbage collector performs some of its collection activities on a separate thread. The setting only applies on multi-processor machines, and defaults to true.

5.7 How can I find out what the garbage collector is doing?

Lots of interesting statistics are exported from the .NET runtime via the '.NET CLR xxx' performance counters. Use Performance Monitor to view them.

5.8 What is the lapsed listener problem?

The lapsed listener problem is one of the primary causes of leaks in .NET applications. It occurs when a subscriber (or 'listener') signs up for a publisher's event, but fails to unsubscribe. The failure to unsubscribe means that the publisher maintains a reference to the subscriber as long as the publisher is alive. For some publishers, this may be the duration of the application.

This situation causes two problems. The obvious problem is the leakage of the subscriber object. The other problem is the performance degredation due to the publisher sending redundant notifications to 'zombie' subscribers.

There are at least a couple of solutions to the problem. The simplest is to make sure the subscriber is unsubscribed from the publisher, typically by adding an Unsubscribe() method to the subscriber. Another solution, documented here by Shawn Van Ness, is to change the publisher to use weak references in its subscriber list.

5.9 When do I need to use GC.KeepAlive?

It's very unintuitive, but the runtime can decide that an object is garbage much sooner than you expect. More specifically, an object can become garbage while a method is executing on the object, which is contrary to most developers' expectations. Chris Brumme explains the issue on his blog. I've taken Chris's code and expanded it into a full app that you can play with if you want to prove to yourself that this is a real problem:

    using System;
   using System.Runtime.InteropServices;

   class Win32
   {
       [DllImport("kernel32.dll")]
       public static extern IntPtr CreateEvent( IntPtr lpEventAttributes,
           bool bManualReset,bool bInitialState, string lpName);

       [DllImport("kernel32.dll", SetLastError=true)]
       public static extern bool CloseHandle(IntPtr hObject);

       [DllImport("kernel32.dll")]
       public static extern bool SetEvent(IntPtr hEvent);
   }

   class EventUser
   {
       public EventUser()
       {
           hEvent = Win32.CreateEvent( IntPtr.Zero, false, false, null );
       }
      
       ~EventUser()
       {
           Win32.CloseHandle( hEvent );
           Console.WriteLine("EventUser finalized");
       }

       public void UseEvent()
       {
           UseEventInStatic( this.hEvent );
       }

       static void UseEventInStatic( IntPtr hEvent )
       {
           //GC.Collect();
           bool bSuccess = Win32.SetEvent( hEvent );
           Console.WriteLine( "SetEvent " + (bSuccess ? "succeeded" : "FAILED!") );
       }

       IntPtr hEvent;
   }

   class App
   {
       static void Main(string[] args)
       {
           EventUser eventUser = new EventUser();
           eventUser.UseEvent();
       }
   }

If you run this code, it'll probably work fine, and you'll get the following output:

    SetEvent succeeded
   EventDemo finalized

However, if you uncomment the GC.Collect() call in the UseEventInStatic() method, you'll get this output:

    EventDemo finalized
   SetEvent FAILED!

(Note that you need to use a release build to reproduce this problem.)

So what's happening here? Well, at the point where UseEvent() calls UseEventInStatic(), a copy is taken of the hEvent field, and there are no further references to the EventUser object anywhere in the code. So as far as the runtime is concerned, the EventUser object is garbage and can be collected. Normally of course the collection won't happen immediately, so you'll get away with it, but sooner or later a collection will occur at the wrong time, and your app will fail.

A solution to this problem is to add a call to GC.KeepAlive(this) to the end of the UseEvent method, as Chris explains.

6. Serialization

6.1 What is serialization?

Serialization is the process of converting an object into a stream of bytes. Deserialization is the opposite process, i.e. creating an object from a stream of bytes. Serialization/Deserialization is mostly used to transport objects (e.g. during remoting), or to persist objects (e.g. to a file or database).

6.2 Does the .NET Framework have in-built support for serialization?

There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.

6.3 I want to serialize instances of my class. Should I use XmlSerializer, SoapFormatter or BinaryFormatter?

It depends. XmlSerializer has severe limitations such as the requirement that the target class has a parameterless constructor, and only public read/write properties and fields can be serialized. However, on the plus side, XmlSerializer has good support for customising the XML document that is produced or consumed. XmlSerializer's features mean that it is most suitable for cross-platform work, or for constructing objects from existing XML documents.

SoapFormatter and BinaryFormatter have fewer limitations than XmlSerializer. They can serialize private fields, for example. However they both require that the target class be marked with the [Serializable] attribute, so like XmlSerializer the class needs to be written with serialization in mind. Also there are some quirks to watch out for - for example on deserialization the constructor of the new object is not invoked.

The choice between SoapFormatter and BinaryFormatter depends on the application. BinaryFormatter makes sense where both serialization and deserialization will be performed on the .NET platform and where performance is important. SoapFormatter generally makes more sense in all other cases, for ease of debugging if nothing else.

6.4 Can I customise the serialization process?

Yes. XmlSerializer supports a range of attributes that can be used to configure serialization for a particular class. For example, a field or property can be marked with the [XmlIgnore] attribute to exclude it from serialization. Another example is the [XmlElement] attribute, which can be used to specify the XML element name to be used for a particular property or field.

Serialization via SoapFormatter/BinaryFormatter can also be controlled to some extent by attributes. For example, the [NonSerialized] attribute is the equivalent of XmlSerializer's [XmlIgnore] attribute. Ultimate control of the serialization process can be acheived by implementing the the ISerializable interface on the class whose instances are to be serialized.

6.5 Why is XmlSerializer so slow?

There is a once-per-process-per-type overhead with XmlSerializer. So the first time you serialize or deserialize an object of a given type in an application, there is a significant delay. This normally doesn't matter, but it may mean, for example, that XmlSerializer is a poor choice for loading configuration settings during startup of a GUI application.

6.6 Why do I get errors when I try to serialize a Hashtable?

XmlSerializer will refuse to serialize instances of any class that implements IDictionary, e.g. Hashtable. SoapFormatter and BinaryFormatter do not have this restriction.

6.7 XmlSerializer is throwing a generic "There was an error reflecting MyClass" error. How do I find out what the problem is?

Look at the InnerException property of the exception that is thrown to get a more specific error message.

6.8 Why am I getting an InvalidOperationException when I serialize an ArrayList?

XmlSerializer needs to know in advance what type of objects it will find in an ArrayList. To specify the type, use the XmlArrayItem attibute like this:

    public class Person
   {
       public string Name;
       public int Age;
   }

   public class Population
   {
       [XmlArrayItem(typeof(Person))] public ArrayList People;
   }

7. Attributes

7.1 What are attributes?

There are at least two types of .NET attribute. The first type I will refer to as a metadata attribute - it allows some data to be attached to a class or method. This data becomes part of the metadata for the class, and (like other class metadata) can be accessed via reflection. An example of a metadata attribute is [serializable], which can be attached to a class and means that instances of the class can be serialized.

    [serializable] public class CTest {}

The other type of attribute is a context attribute. Context attributes use a similar syntax to metadata attributes but they are fundamentally different. Context attributes provide an interception mechanism whereby instance activation and method calls can be pre- and/or post-processed. If you have encountered Keith Brown's universal delegator you'll be familiar with this idea.

7.2 Can I create my own metadata attributes?

Yes. Simply derive a class from System.Attribute and mark it with the AttributeUsage attribute. For example:

    [AttributeUsage(AttributeTargets.Class)]
   public class InspiredByAttribute : System.Attribute
   {
       public string InspiredBy;
  
       public InspiredByAttribute( string inspiredBy )
       {
           InspiredBy = inspiredBy;
       }
   }


   [InspiredBy("Andy Mc's brilliant .NET FAQ")]
   class CTest
   {
   }


   class CApp
   {
       public static void Main()
       {       
           object[] atts = typeof(CTest).GetCustomAttributes(true);

           foreach( object att in atts )
               if( att is InspiredByAttribute )
                   Console.WriteLine( "Class CTest was inspired by {0}", ((InspiredByAttribute)att).InspiredBy  );
       }
   }

7.3 Can I create my own context attibutes?

Yes. Take a look at Peter Drayton's Tracehook.NET.

8. Code Access Security

8.1 What is Code Access Security (CAS)?

CAS is the part of the .NET security model that determines whether or not code is allowed to run, and what resources it can use when it is running. For example, it is CAS that will prevent a .NET web applet from formatting your hard disk.

8.2 How does CAS work?

The CAS security policy revolves around two key concepts - code groups and permissions. Each .NET assembly is a member of a particular code group, and each code group is granted the permissions specified in a named permission set.

For example, using the default security policy, a control downloaded from a web site belongs to the 'Zone - Internet' code group, which adheres to the permissions defined by the 'Internet' named permission set. (Naturally the 'Internet' named permission set represents a very restrictive range of permissions.)

8.3 Who defines the CAS code groups?

Microsoft defines some default ones, but you can modify these and even create your own. To see the code groups defined on your system, run 'caspol -lg' from the command-line. On my system it looks like this:

   Level = Machine

  Code Groups:

  1.  All code: Nothing
  1.1.  Zone - MyComputer: FullTrust
     1.1.1.  Honor SkipVerification requests: SkipVerification
  1.2.  Zone - Intranet: LocalIntranet
  1.3.  Zone - Internet: Internet
  1.4.  Zone - Untrusted: Nothing
  1.5.  Zone - Trusted: Internet
  1.6.  StrongName -
0024000004800000940000000602000000240000525341310004000003
000000CFCB3291AA715FE99D40D49040336F9056D7886FED46775BC7BB5430BA4444FEF8348EBD06
F962F39776AE4DC3B7B04A7FE6F49F25F740423EBF2C0B89698D8D08AC48D69CED0FC8F83B465E08
07AC11EC1DCC7D054E807A43336DDE408A5393A48556123272CEEEE72F1660B71927D38561AABF5C
AC1DF1734633C602F8F2D5: Everything

Note the hierarchy of code groups - the top of the hierarchy is the most general ('All code'), which is then sub-divided into several groups, each of which in turn can be sub-divided. Also note that (somewhat counter-intuitively) a sub-group can be associated with a more permissive permission set than its parent.

8.4 How do I define my own code group?

Use caspol. For example, suppose you trust code from www.mydomain.com and you want it have full access to your system, but you want to keep the default restrictions for all other internet sites. To achieve this, you would add a new code group as a sub-group of the 'Zone - Internet' group, like this:

 caspol -ag 1.3 -site www.mydomain.com FullTrust

Now if you run caspol -lg you will see that the new group has been added as group 1.3.1:

 ...
   1.3.  Zone - Internet: Internet
      1.3.1.  Site - www.mydomain.com: FullTrust
...

Note that the numeric label (1.3.1) is just a caspol invention to make the code groups easy to manipulate from the command-line. The underlying runtime never sees it.

8.5 How do I change the permission set for a code group?

Use caspol. If you are the machine administrator, you can operate at the 'machine' level - which means not only that the changes you make become the default for the machine, but also that users cannot change the permissions to be more permissive. If you are a normal (non-admin) user you can still modify the permissions, but only to make them more restrictive. For example, to allow intranet code to do what it likes you might do this:

 caspol -cg 1.2 FullTrust

Note that because this is more permissive than the default policy (on a standard system), you should only do this at the machine level - doing it at the user level will have no effect.

8.6 Can I create my own permission set?

Yes. Use caspol -ap, specifying an XML file containing the permissions in the permission set. To save you some time, here is a sample file corresponding to the 'Everything' permission set - just edit to suit your needs. When you have edited the sample, add it to the range of available permission sets like this:

 caspol -ap samplepermset.xml

Then, to apply the permission set to a code group, do something like this:

 caspol -cg 1.3 SamplePermSet

(By default, 1.3 is the 'Internet' code group)

8.7 I'm having some trouble with CAS. How can I troubleshoot the problem?

Caspol has a couple of options that might help. First, you can ask caspol to tell you what code group an assembly belongs to, using caspol -rsg. Similarly, you can ask what permissions are being applied to a particular assembly using caspol -rsp.

8.8 I can't be bothered with CAS. Can I turn it off?

Yes, as long as you are an administrator. Just run:

 caspol -s off

9. Intermediate Language (IL)

9.1 Can I look at the IL for an assembly?

Yes. MS supply a tool called Ildasm that can be used to view the metadata and IL for an assembly.

9.2 Can source code be reverse-engineered from IL?

Yes, it is often relatively straightforward to regenerate high-level source from IL. Lutz Roeder's Reflector does a very good job of turning IL into C# or VB.NET.

9.3 How can I stop my code being reverse-engineered from IL?

You can buy an IL obfuscation tool. These tools work by 'optimising' the IL in such a way that reverse-engineering becomes much more difficult.

Of course if you are writing web services then reverse-engineering is not a problem as clients do not have access to your IL.

9.4 Can I write IL programs directly?

Yes. Peter Drayton posted this simple example to the DOTNET mailing list:

    .assembly MyAssembly {}
   .class MyApp {
     .method static void Main() {
       .entrypoint
       ldstr      "Hello, IL!"
       call       void System.Console::WriteLine(class System.Object)
       ret
     }
   }

Just put this into a file called hello.il, and then run ilasm hello.il. An exe assembly will be generated.

9.5 Can I do things in IL that I can't do in C#?

Yes. A couple of simple examples are that you can throw exceptions that are not derived from System.Exception, and you can have non-zero-based arrays.

10. Implications for COM

10.1 Does .NET replace COM?

This subject causes a lot of controversy, as you'll see if you read the mailing list archives. Take a look at the following two threads:

http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&D=0&P=68241
http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&P=R60761

The bottom line is that .NET has its own mechanisms for type interaction, and they don't use COM. No IUnknown, no IDL, no typelibs, no registry-based activation. This is mostly good, as a lot of COM was ugly. Generally speaking, .NET allows you to package and use components in a similar way to COM, but makes the whole thing a bit easier.

10.2 Is DCOM dead?

Pretty much, for .NET developers. The .NET Framework has a new remoting model which is not based on DCOM. DCOM was pretty much dead anyway, once firewalls became widespread and Microsoft got SOAP fever. Of course DCOM will still be used in interop scenarios.

10.3 Is COM+ dead?

Not immediately. The approach for .NET 1.0 was to provide access to the existing COM+ services (through an interop layer) rather than replace the services with native .NET ones. Various tools and attributes were provided to make this as painless as possible. Over time it is expected that interop will become more seamless - this may mean that some services become a core part of the CLR, and/or it may mean that some services will be rewritten as managed code which runs on top of the CLR.

For more on this topic, search for postings by Joe Long in the archives - Joe is the MS group manager for COM+. Start with this message:

http://discuss.develop.com/archives/wa.exe?A2=ind0007&L=DOTNET&P=R68370

10.4 Can I use COM components from .NET programs?

Yes. COM components are accessed from the .NET runtime via a Runtime Callable Wrapper (RCW). This wrapper turns the COM interfaces exposed by the COM component into .NET-compatible interfaces. For oleautomation interfaces, the RCW can be generated automatically from a type library. For non-oleautomation interfaces, it may be necessary to develop a custom RCW which manually maps the types exposed by the COM interface to .NET-compatible types.

Here's a simple example for those familiar with ATL. First, create an ATL component which implements the following IDL:

    import "oaidl.idl";
   import "ocidl.idl";

   [
       object,
       uuid(EA013F93-487A-4403-86EC-FD9FEE5E6206),
       helpstring("ICppName Interface"),
       pointer_default(unique),
       oleautomation
   ]

   interface ICppName : IUnknown
   {
       [helpstring("method SetName")] HRESULT SetName([in] BSTR name);
       [helpstring("method GetName")] HRESULT GetName([out,retval] BSTR *pName );
   };

   [
       uuid(F5E4C61D-D93A-4295-A4B4-2453D4A4484D),
       version(1.0),
       helpstring("cppcomserver 1.0 Type Library")
   ]
   library CPPCOMSERVERLib
   {
       importlib("stdole32.tlb");
       importlib("stdole2.tlb");
       [
           uuid(600CE6D9-5ED7-4B4D-BB49-E8D5D5096F70), 
           helpstring("CppName Class")
       ]
       coclass CppName
       {
           [default] interface ICppName;
       };
   };

When you've built the component, you should get a typelibrary. Run the TLBIMP utility on the typelibary, like this:

    tlbimp cppcomserver.tlb

If successful, you will get a message like this:

    Typelib imported successfully to CPPCOMSERVERLib.dll

You now need a .NET client - let's use C#. Create a .cs file containing the following code:

    using System;
   using CPPCOMSERVERLib;

   public class MainApp
   {
       static public void Main()
       {
           CppName cppname = new CppName();
           cppname.SetName( "bob" );
           Console.WriteLine( "Name is " + cppname.GetName() );
       }
   }

Compile the C# code like this:

    csc /r:cppcomserverlib.dll csharpcomclient.cs

Note that the compiler is being told to reference the DLL we previously generated from the typelibrary using TLBIMP. You should now be able to run csharpcomclient.exe, and get the following output on the console:

    Name is bob

10.5 Can I use .NET components from COM programs?

Yes. .NET components are accessed from COM via a COM Callable Wrapper (CCW). This is similar to a RCW (see previous question), but works in the opposite direction. Again, if the wrapper cannot be automatically generated by the .NET development tools, or if the automatic behaviour is not desirable, a custom CCW can be developed. Also, for COM to 'see' the .NET component, the .NET component must be registered in the registry.

Here's a simple example. Create a C# file called testcomserver.cs and put the following in it:

    using System;
   using System.Runtime.InteropServices;

   namespace AndyMc
   {
       [ClassInterface(ClassInterfaceType.AutoDual)]
       public class CSharpCOMServer
       {
           public CSharpCOMServer() {}
           public void SetName( string name ) { m_name = name; }
           public string GetName() { return m_name; } 
           private string m_name;
       }         
   }

Then compile the .cs file as follows:

    csc /target:library testcomserver.cs

You should get a dll, which you register like this:

    regasm testcomserver.dll /tlb:testcomserver.tlb /codebase

Now you need to create a client to test your .NET COM component. VBScript will do - put the following in a file called comclient.vbs:

    Dim dotNetObj
   Set dotNetObj = CreateObject("AndyMc.CSharpCOMServer")
   dotNetObj.SetName ("bob")
   MsgBox "Name is " & dotNetObj.GetName()

and run the script like this:

    wscript comclient.vbs

And hey presto you should get a message box displayed with the text "Name is bob".

An alternative to the approach above it to use the dm.net moniker developed by Jason Whittington and Don Box.

10.6 Is ATL redundant in the .NET world?

Yes. ATL will continue to be valuable for writing COM components for some time, but it has no place in the .NET world.

11. Threads

11.1 How do I spawn a thread?

Create an instance of a System.Threading.Thread object, passing it an instance of a ThreadStart delegate that will be executed on the new thread. For example:

    class MyThread
   {
       public MyThread( string initData )
       {
           m_data = initData;
           m_thread = new Thread( new ThreadStart(ThreadMain) );   
           m_thread.Start();   
       }

       // ThreadMain() is executed on the new thread.
       private void ThreadMain()
       {   
           Console.WriteLine( m_data );
       }

       public void WaitUntilFinished()
       {
           m_thread.Join();
       }   

       private Thread m_thread;
       private string m_data;
   }

In this case creating an instance of the MyThread class is sufficient to spawn the thread and execute the MyThread.ThreadMain() method:

    MyThread t = new MyThread( "Hello, world." );
   t.WaitUntilFinished();

11.2 How do I stop a thread?

There are several options. First, you can use your own communication mechanism to tell the ThreadStart method to finish. Alternatively the Thread class has in-built support for instructing the thread to stop. The two principle methods are Thread.Interrupt() and Thread.Abort(). The former will cause a ThreadInterruptedException to be thrown on the thread when it next goes into a WaitJoinSleep state. In other words, Thread.Interrupt is a polite way of asking the thread to stop when it is no longer doing any useful work. In contrast, Thread.Abort() throws a ThreadAbortException regardless of what the thread is doing. Furthermore, the ThreadAbortException cannot normally be caught (though the ThreadStart's finally method will be executed). Thread.Abort() is a heavy-handed mechanism which should not normally be required.

11.3 How do I use the thread pool?

By passing an instance of a WaitCallback delegate to the ThreadPool.QueueUserWorkItem() method

    class CApp
   {
       static void Main()
       {
           string s = "Hello, World";
           ThreadPool.QueueUserWorkItem( new WaitCallback( DoWork ), s );
      
           Thread.Sleep( 1000 );    // Give time for work item to be executed
       }

       // DoWork is executed on a thread from the thread pool.
       static void DoWork( object state )
       {
           Console.WriteLine( state );
       }
   }

11.4 How do I know when my thread pool work item has completed?

There is no way to query the thread pool for this information. You must put code into the WaitCallback method to signal that it has completed. Events are useful for this.

11.5 How do I prevent concurrent access to my data?

Each object has a concurrency lock (critical section) associated with it. The System.Threading.Monitor.Enter/Exit methods are used to acquire and release this lock. For example, instances of the following class only allow one thread at a time to enter method f():

    class C
   {
       public void f()
       {
           try
           {
               Monitor.Enter(this);
               ...
           }
           finally
           {
               Monitor.Exit(this);
           }
       }
   }

C# has a 'lock' keyword which provides a convenient shorthand for the code above:

    class C
   {
       public void f()
       {
           lock(this)
           {
               ...
           }
       }
   }

Note that calling Monitor.Enter(myObject) does NOT mean that all access to myObject is serialized. It means that the synchronisation lock associated with myObject has been acquired, and no other thread can acquire that lock until Monitor.Exit(o) is called. In other words, this class is functionally equivalent to the classes above:

    class C
   {
       public void f()
       {
           lock( m_object )
           {
               ...
           }
       }
      
       private m_object = new object();
   }

Actually, it could be argued that this version of the code is superior, as the lock is totally encapsulated within the class, and not accessible to the user of the object.

11.6 Should I use ReaderWriterLock instead of Monitor.Enter/Exit?

Maybe, but be careful. ReaderWriterLock is used to allow multiple threads to read from a data source, while still granting exclusive access to a single writer thread. This makes sense for data access that is mostly read-only, but there are some caveats. First, ReaderWriterLock is relatively poor performing compared to Monitor.Enter/Exit, which offsets some of the benefits. Second, you need to be very sure that the data structures you are accessing fully support multithreaded read access. Finally, there is apparently a bug in the v1.1 ReaderWriterLock that can cause starvation for writers when there are a large number of readers.

Ian Griffiths has some interesting discussion on ReaderWriterLock here and here.

12. Tracing

12.1 Is there built-in support for tracing/logging?

Yes, in the System.Diagnostics namespace. There are two main classes that deal with tracing - Debug and Trace. They both work in a similar way - the difference is that tracing from the Debug class only works in builds that have the DEBUG symbol defined, whereas tracing from the Trace class only works in builds that have the TRACE symbol defined. Typically this means that you should use System.Diagnostics.Trace.WriteLine for tracing that you want to work in debug and release builds, and System.Diagnostics.Debug.WriteLine for tracing that you want to work only in debug builds.

12.2 Can I redirect tracing to a file?

Yes. The Debug and Trace classes both have a Listeners property, which is a collection of sinks that receive the tracing that you send via Debug.WriteLine and Trace.WriteLine respectively. By default the Listeners collection contains a single sink, which is an instance of the DefaultTraceListener class. This sends output to the Win32 OutputDebugString() function and also the System.Diagnostics.Debugger.Log() method. This is useful when debugging, but if you're trying to trace a problem at a customer site, redirecting the output to a file is more appropriate. Fortunately, the TextWriterTraceListener class is provided for this purpose.

Here's how to use the TextWriterTraceListener class to redirect Trace output to a file:

    Trace.Listeners.Clear();
   FileStream fs = new FileStream( @"c:\log.txt", FileMode.Create, FileAccess.Write );
   Trace.Listeners.Add( new TextWriterTraceListener( fs ) );

   Trace.WriteLine( @"This will be writen to c:\log.txt!" );
   Trace.Flush();

Note the use of Trace.Listeners.Clear() to remove the default listener. If you don't do this, the output will go to the file and OutputDebugString(). Typically this is not what you want, because OutputDebugString() imposes a big performance hit.

12.3 Can I customise the trace output?

Yes. You can write your own TraceListener-derived class, and direct all output through it. Here's a simple example, which derives from TextWriterTraceListener (and therefore has in-built support for writing to files, as shown above) and adds timing information and the thread ID for each trace line:

    class MyListener : TextWriterTraceListener
   {
       public MyListener( Stream s ) : base(s)
       {
       }

       public override void WriteLine( string s )
       {
           Writer.WriteLine( "{0:D8} [{1:D4}] {2}",
               Environment.TickCount - m_startTickCount,
               AppDomain.GetCurrentThreadId(),
               s );
       }

       protected int m_startTickCount = Environment.TickCount;
   }

(Note that this implementation is not complete - the TraceListener.Write method is not overridden for example.)

The beauty of this approach is that when an instance of MyListener is added to the Trace.Listeners collection, all calls to Trace.WriteLine() go through MyListener, including calls made by referenced assemblies that know nothing about the MyListener class.

12.4 Are there any third party logging components available?

Log4net is a port of the established log4j Java logging component.

13. Miscellaneous

13.1 How does .NET remoting work?

.NET remoting involves sending messages along channels. Two of the standard channels are HTTP and TCP. TCP is intended for LANs only - HTTP can be used for LANs or WANs (internet).

Support is provided for multiple message serializarion formats. Examples are SOAP (XML-based) and binary. By default, the HTTP channel uses SOAP (via the .NET runtime Serialization SOAP Formatter), and the TCP channel uses binary (via the .NET runtime Serialization Binary Formatter). But either channel can use either serialization format.

There are a number of styles of remote access:

  • SingleCall. Each incoming request from a client is serviced by a new object. The object is thrown away when the request has finished.
  • Singleton. All incoming requests from clients are processed by a single server object.
  • Client-activated object. This is the old stateful (D)COM model whereby the client receives a reference to the remote object and holds that reference (thus keeping the remote object alive) until it is finished with it.

Distributed garbage collection of objects is managed by a system called 'leased based lifetime'. Each object has a lease time, and when that time expires the object is disconnected from the .NET runtime remoting infrastructure. Objects have a default renew time - the lease is renewed when a successful call is made from the client to the object. The client can also explicitly renew the lease.

If you're interested in using XML-RPC as an alternative to SOAP, take a look at Charles Cook's XML-RPC.Net.

13.2 How can I get at the Win32 API from a .NET program?

Use P/Invoke. This uses similar technology to COM Interop, but is used to access static DLL entry points instead of COM objects. Here is an example of C# calling the Win32 MessageBox function:

    using System;
   using System.Runtime.InteropServices;

   class MainApp
   {
       [DllImport("user32.dll", EntryPoint="MessageBox", SetLastError=true, CharSet=CharSet.Auto)]    
       public static extern int MessageBox(int hWnd, String strMessage, String strCaption, uint uiType);
  
       public static void Main()
       {
           MessageBox( 0, "Hello, this is PInvoke in operation!", ".NET", 0 );
       }
   }

Pinvoke.net is a great resource for off-the-shelf P/Invoke signatures.

13.3 How do I write to the application configuration file at runtime?

You don't. See http://www.interact-sw.co.uk/iangblog/2004/11/25/savingconfig.

13.4 What is the difference between an event and a delegate?

An event is just a wrapper for a multicast delegate. Adding a public event to a class is almost the same as adding a public multicast delegate field. In both cases, subscriber objects can register for notifications, and in both cases the publisher object can send notifications to the subscribers. However, a public multicast delegate has the undesirable property that external objects can invoke the delegate, something we'd normally want to restrict to the publisher. Hence events - an event adds public methods to the containing class to add and remove receivers, but does not make the invocation mechanism public.

See this post by Julien Couvreur for more discussion.

13.5 What size is a .NET object?

Each instance of a reference type has two fields maintained by the runtime - a method table pointer and a sync block. These are 4 bytes each on a 32-bit system, making a total of 8 bytes per object overhead. Obviously the instance data for the type must be added to this to get the overall size of the object. So, for example, instances of the following class are 12 bytes each:

    class MyInt
   {
       ...
       private int x;
   }

However, note that with the current implementation of the CLR there seems to be a minimum object size of 12 bytes, even for classes with no data (e.g. System.Object).

Values types have no equivalent overhead.

13.6 Will my .NET app run on 64-bit Windows?

64-bit (x64) versions of Windows support both 32-bit and 64-bit processes, and corresponding 32-bit and 64-bit versions of .NET 2.0. (.NET 1.1 is 32-bit only).

.NET 1.x apps automatically run as 32-bit processes on 64-bit Windows.

.NET 2.0 apps can either run as 32-bit processes or as 64-bit processes. The OS decides which to use based on the PE header of the executable. The flags in the PE header are controlled via the compiler /platform switch, which allows the target of the app to be specified as 'x86', 'x64' or 'any cpu'. Normally you specify 'any cpu', and your app will run as 32-bit on 32-bit Windows and 64-bit on 64-bit Windows. However if you have some 32-bit native code in your app (loaded via COM interop, for example), you will need to specify 'x86', which will force 64-bit Windows to load your app in a 32-bit process. You can also tweak the 32-bit flag in the PE header using the SDK corflags utility.

Some more explanation here:

http://blogs.msdn.com/gauravseth/archive/2006/03/07/545104.aspx
http://blogs.msdn.com/joshwil/archive/2005/04/08/406567.aspx
http://msdn.microsoft.com/netframework/programming/64bit/gettingstarted/

13.7 What is reflection?

All .NET compilers produce metadata about the types defined in the modules they produce. This metadata is packaged along with the module (modules in turn are packaged together in assemblies), and can be accessed by a mechanism called reflection. The System.Reflection namespace contains classes that can be used to interrogate the types for a module/assembly.

Using reflection to access .NET metadata is very similar to using ITypeLib/ITypeInfo to access type library data in COM, and it is used for similar purposes - e.g. determining data type sizes for marshaling data across context/process/machine boundaries.

Reflection can also be used to dynamically invoke methods (see System.Type.InvokeMember), or even create types dynamically at run-time (see System.Reflection.Emit.TypeBuilder).

14. .NET 2.0

14.1 What are the new features of .NET 2.0?

Generics, anonymous methods, partial classes, iterators, property visibility (separate visibility for get and set) and static classes. See http://msdn.microsoft.com/msdnmag/issues/04/05/C20/default.aspx for more information about these features.

14.2 What are the new 2.0 features useful for?

Generics are useful for writing efficient type-independent code, particularly where the types might include value types. The obvious application is container classes, and the .NET 2.0 class library includes a suite of generic container classes in the System.Collections.Generic namespace. Here's a simple example of a generic container class being used:

    List myList = new List();
   myList.Add( 10 );

Anonymous methods reduce the amount of code you have to write when using delegates, and are therefore especially useful for GUI programming. Here's an example

    AppDomain.CurrentDomain.ProcessExit += delegate { Console.WriteLine("Process ending ..."); };

Partial classes is a useful feature for separating machine-generated code from hand-written code in the same class, and will therefore be heavily used by development tools such as Visual Studio.

Iterators reduce the amount of code you need to write to implement IEnumerable/IEnumerator. Here's some sample code:

    static void Main()
   {
       RandomEnumerator re = new RandomEnumerator( 5 );
       foreach( double r in re )
           Console.WriteLine( r );
       Console.Read();
   }

   class RandomEnumerator : IEnumerable
   {
       public RandomEnumerator(int size) { m_size = size; }

       public IEnumerator GetEnumerator()
       {
           Random rand = new Random();
           for( int i=0; i < m_size; i++ )
               yield return rand.NextDouble();
       }

       int m_size = 0;
   }

The use of 'yield return' is rather strange at first sight. It effectively synthethises an implementation of IEnumerator, something we had to do manually in .NET 1.x.

14.3 What's the problem with .NET generics?

.NET generics work great for container classes. But what about other uses? Well, it turns out that .NET generics have a major limitation - they require the type parameter to be constrained. For example, you cannot do this:

    static class Disposer
   {
       public static void Dispose(T obj) { obj.Dispose(); }
   }

The C# compiler will refuse to compile this code, as the type T has not been constrained, and therefore only supports the methods of System.Object. Dispose is not a method on System.Object, so the compilation fails. To fix this code, we need to add a where clause, to reassure the compiler that our type T does indeed have a Dispose method

    static class Disposer where T : IDisposable
   {
       public static void Dispose(T obj) { obj.Dispose(); }
   }

The problem is that the requirement for explicit contraints is very limiting. We can use constraints to say that T implements a particular interface, but we can't dilute that to simply say that T implements a particular method. Contrast this with C++ templates (for example), where no constraint at all is required - it is assumed (and verified at compile time) that if the code invokes the Dispose() method on a type, then the type will support the method.

In fact, after writing generic code with interface constraints, we quickly see that we haven't gained much over non-generic interface-based programming. For example, we can easily rewrite the Disposer class without generics:

    static class Disposer
   {
       public static void Dispose( IDisposable obj ) { obj.Dispose(); }
   }

For more on this topic, start by reading the following articles:

Bruce Eckel: http://www.mindview.net/WebLog/log-0050
Ian Griffiths: http://www.interact-sw.co.uk/iangblog/2004/03/14/generics
Charles Cook: http://www.cookcomputing.com/blog/archives/000425.html

14.4 What's new in the .NET 2.0 class library?

Here is a selection of new features in the .NET 2.0 class library:

  • Generic collections in the System.Collections.Generic namespace.
  • The System.Nullable type. (Note that C# has special syntax for this type, e.g. int? is equivalent to Nullable)
  • The GZipStream and DeflateStream classes in the System.IO.Compression namespace.
  • The Semaphore class in the System.Threading namespace.
  • Wrappers for DPAPI in the form of the ProtectedData and ProtectedMemory classes in the System.Security.Cryptography namespace.
  • The IPC remoting channel in the System.Runtime.Remoting.Channels.Ipc namespace, for optimised intra-machine communication.

and many, many more. See http://msdn2.microsoft.com/en-us/library/t357fb32(en-US,VS.80).aspx for a comprehensive list of changes.

15. Resources

15.1 Recommended books

I recommend the following books, either because I personally like them, or because I think they are well regarded by other .NET developers. (Note that I get a commission from Amazon if you buy a book after following one of these links.)

15.2 Web sites

15.3 Blogs

The following Weblogs ('blogs') have regular .NET content:

15.4 Free tools

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.NET And Asp .Net Interview Questions And Answers

How many languages .NET is supporting now?
When .NET was introduced it came with several languages. VB.NET, C#, COBOL and Perl, etc. 44 languages are supported.

How is .NET able to support multiple languages?
A language should comply with the Common Language Runtime standard to become a .NET language. In .NET, code is compiled to Microsoft Intermediate Language (MSIL for short). This is called as Managed Code. This Managed code is run in .NET environment. So after compilation to this IL the language is not a barrier. A code can call or use a function written in another language.

How ASP .NET different from ASP?
Scripting is separated from the HTML, Code is compiled as a DLL, these DLLs can be executed on the server.

What is smart navigation?
The cursor position is maintained when the page gets refreshed due to the server side validation and the page gets refreshed.

What is view state?
The web is stateless. But in ASP.NET, the state of a page is maintained in the in the page itself automatically. How? The values are encrypted and saved in hidden controls. this is done automatically by the ASP.NET. This can be switched off / on for a single control

How do you validate the controls in an ASP .NET page?
Using special validation controls that are meant for this. We have Range Validator, Email Validator.

Can the validation be done in the server side? Or this can be done only in the Client side?
Client side is done by default. Server side validation is also possible. We can switch off the client side and server side can be done.

How to manage pagination in a page?
Using pagination option in DataGrid control. We have to set the number of records for a page, then it takes care of pagination by itself.

What is ADO .NET and what is difference between ADO and ADO.NET?
ADO.NET is stateless mechanism. I can treat the ADO.Net as a separate in-memory database where in I can use relationships between the tables and select insert and updates to the database. I can update the actual database as a batch.

Explain the differences between Server-side and Client-side code?

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Server side scripting means that all the script will be executed by the server and interpreted as needed. ASP doesn’t have some of the functionality like sockets, uploading, etc. For these you have to make a custom components usually in VB or VC++. Client side scripting means that the script will be executed immediately in the browser such as form field validation, clock, email validation, etc. Client side scripting is usually done in VBScript or JavaScript. Download time, browser compatibility, and visible code - since JavaScript and VBScript code is included in the HTML page, then anyone can see the code by viewing the page source. Also a possible security hazards for the client computer.

What type of code (server or client) is found in a Code-Behind class?
C#

Should validation (did the user enter a real date) occur server-side or client-side? Why?
Client-side validation because there is no need to request a server side date when you could obtain a date from the client machine.

What does the "EnableViewState" property do? Why would I want it on or off?
Enable ViewState turns on the automatic state management feature that enables server controls to re-populate their values on a round trip without requiring you to write any code. This feature is not free however, since the state of a control is passed to and from the server in a hidden form field. You should be aware of when ViewState is helping you and when it is not. For example, if you are binding a control to data on every round trip (as in the datagrid example in tip #4), then you do not need the control to maintain it’s view state, since you will wipe out any re-populated data in any case. ViewState is enabled for all server controls by default. To disable it, set the EnableViewState property of the control to false.

What is the difference between Server.Transfer and Response.Redirect?
Why would I choose one over the other? Server.Transfer() : client is shown as it is on the requesting page only, but the all the content is of the requested page. Data can be persist across the pages using Context.Item collection, which is one of the best way to transfer data from one page to another keeping the page state alive. Response.Dedirect() :client know the physical location (page name and query string as well). Context.Items loses the persistence when navigate to destination page. In earlier versions of IIS, if we wanted to send a user to a new Web page, the only option we had was Response.Redirect. While this method does accomplish our goal, it has several important drawbacks. The biggest problem is that this method causes each page to be treated as a separate transaction. Besides making it difficult to maintain your transactional integrity, Response.Redirect introduces some additional headaches. First, it prevents good encapsulation of code. Second, you lose access to all of the properties in the Request object. Sure, there are workarounds, but they’re difficult. Finally, Response.Redirect necessitates a round trip to the client, which, on high-volume sites, causes scalability problems. As you might suspect, Server.Transfer fixes all of these problems. It does this by performing the transfer on the server without requiring a roundtrip to the client.

Can you give an example of when it would be appropriate to use a web service as opposed to a non-serviced .NET component?
When to Use Web Services:
* Communicating through a Firewall When building a distributed application with 100s/1000s of users spread over multiple locations, there is always the problem of communicating between client and server because of firewalls and proxy servers. Exposing your middle tier components as Web Services and invoking the directly from a Windows UI is a very valid option.

* Application Integration When integrating applications written in various languages and running on disparate systems. Or even applications running on the same platform that have been written by separate vendors.

* Business-to-Business Integration This is an enabler for B2B integration which allows one to expose vital business processes to authorized supplier and customers. An example would be exposing electronic ordering and invoicing, allowing customers to send you purchase orders and suppliers to send you invoices electronically.

* Software Reuse This takes place at multiple levels. Code Reuse at the Source code level or binary component-based reuse. The limiting factor here is that you can reuse the code but not the data behind it. Webservice overcome this limitation. A scenario could be when you are building an app that aggregates the functionality of several other Applications. Each of these functions could be performed by individual apps, but there is value in perhaps combining the multiple apps to present a unified view in a Portal or Intranet.

* When not to use Web Services: Single machine Applications When the apps are running on the same machine and need to communicate with each other use a native API. You also have the options of using component technologies such as COM or .NET Components as there is very little overhead.

* Homogeneous Applications on a LAN If you have Win32 or Winforms apps that want to communicate to their server counterpart. It is much more efficient to use DCOM in the case of Win32 apps and .NET Remoting in the case of .NET Apps.

Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?
In ADO, the in-memory representation of data is the RecordSet. In ADO.NET, it is the dataset. There are important differences between them.

* A RecordSet looks like a single table. If a recordset is to contain data from multiple database tables, it must use a JOIN query, which assembles the data from the various database tables into a single result table. In contrast, a dataset is a collection of one or more tables. The tables within a dataset are called data tables; specifically, they are DataTable objects. If a dataset contains data from multiple database tables, it will typically contain multiple DataTable objects. That is, each DataTable object typically corresponds to a single database table or view. In this way, a dataset can mimic the structure of the underlying database. A dataset usually also contains relationships. A relationship within a dataset is analogous to a foreign-key relationship in a database —that is, it associates rows of the tables with each other. For example, if a dataset contains a table about investors and another table about each investor’s stock purchases, it could also contain a relationship connecting each row of the investor table with the corresponding rows of the purchase table. Because the dataset can hold multiple, separate tables and maintain information about relationships between them, it can hold much richer data structures than a recordset, including self-relating tables and tables with many-to-many relationships.

* In ADO you scan sequentially through the rows of the recordset using the ADO MoveNext method. In ADO.NET, rows are represented as collections, so you can loop through a table as you would through any collection, or access particular rows via ordinal or primary key index. DataRelation objects maintain information about master and detail records and provide a method that allows you to get records related to the one you are working with. For example, starting from the row of the Investor table for "Nate Sun," you can navigate to the set of rows of the Purchase table describing his purchases. A cursor is a database element that controls record navigation, the ability to update data, and the visibility of changes made to the database by other users. ADO.NET does not have an inherent cursor object, but instead includes data classes that provide the functionality of a traditional cursor. For example, the functionality of a forward-only, read-only cursor is available in the ADO.NET DataReader object. For more information about cursor functionality, see Data Access Technologies.

* Minimized Open Connections: In ADO.NET you open connections only long enough to perform a database operation, such as a Select or Update. You can read rows into a dataset and then work with them without staying connected to the data source. In ADO the recordset can provide disconnected access, but ADO is designed primarily for connected access. There is one significant difference between disconnected processing in ADO and ADO.NET. In ADO you communicate with the database by making calls to an OLE DB provider. In ADO.NET you communicate with the database through a data adapter (an OleDbDataAdapter, SqlDataAdapter, OdbcDataAdapter, or OracleDataAdapter object), which makes calls to an OLE DB provider or the APIs provided by the underlying data source. The important difference is that in ADO.NET the data adapter allows you to control how the changes to the dataset are transmitted to the database — by optimizing for performance, performing data validation checks, or adding any other extra processing. Data adapters, data connections, data commands, and data readers are the components that make up a .NET Framework data provider. Microsoft and third-party providers can make available other .NET Framework data providers that can be integrated into Visual Studio.

* Sharing Data Between Applications. Transmitting an ADO.NET dataset between applications is much easier than transmitting an ADO disconnected recordset. To transmit an ADO disconnected recordset from one component to another, you use COM marshalling. To transmit data in ADO.NET, you use a dataset, which can transmit an XML stream.

* Richer data types.COM marshalling provides a limited set of data types — those defined by the COM standard. Because the transmission of datasets in ADO.NET is based on an XML format, there is no restriction on data types. Thus, the components sharing the dataset can use whatever rich set of data types they would ordinarily use.

* Performance. Transmitting a large ADO recordset or a large ADO.NET dataset can consume network resources; as the amount of data grows, the stress placed on the network also rises. Both ADO and ADO.NET let you minimize which data is transmitted. But ADO.NET offers another performance advantage, in that ADO.NET does not require data-type conversions. ADO, which requires COM marshalling to transmit records sets among components, does require that ADO data types be converted to COM data types.

* Penetrating Firewalls.A firewall can interfere with two components trying to transmit disconnected ADO recordsets. Remember, firewalls are typically configured to allow HTML text to pass, but to prevent system-level requests (such as COM marshalling) from passing.

Can you give an example of what might be best suited to place in the Application_Start and Session_Start subroutines?
The Application_Start event is guaranteed to occur only once throughout the lifetime of the application. It’s a good place to initialize global variables. For example, you might want to retrieve a list of products from a database table and place the list in application state or the Cache object. SessionStateModule exposes both Session_Start and Session_End events.

If I’m developing an application that must accomodate multiple security levels though secure login and my ASP.NET web appplication is spanned across three web-servers (using round-robbin load balancing) what would be the best approach to maintain login-in state for the users?

What are ASP.NET Web Forms? How is this technology different than what is available though ASP?

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Web Forms are the heart and soul of ASP.NET. Web Forms are the User Interface (UI) elements that give your Web applications their look and feel. Web Forms are similar to Windows Forms in that they provide properties, methods, and events for the controls that are placed onto them. However, these UI elements render themselves in the appropriate markup language required by the request, e.g. HTML. If you use Microsoft Visual Studio .NET, you will also get the familiar drag-and-drop interface used to create your UI for your Web application.

How does VB.NET/C# achieve polymorphism?
By using Abstract classes/functions.

Can you explain what inheritance is and an example of when you might use it?
Inheritance is a fundamental feature of an object oriented system and it is simply the ability to inherit data and functionality from a parent object. Rather than developing new objects from scratch, new code can be based on the work of other programmers, adding only new features that are needed.

How would you implement inheritance using VB.NET/C#?
When we set out to implement a class using inheritance, we must first start with an existing class from which we will derive our new subclass. This existing class, or base class, may be part of the .NET system class library framework, it may be part of some other application or .NET assembly, or we may create it as part of our existing application. Once we have a base class, we can then implement one or more subclasses based on that base class. Each of our subclasses will automatically have all of the methods, properties, and events of that base class ? including the implementation behind each method, property, and event. Our subclass can add new methods, properties, and events of its own - extending the original interface with new functionality. Additionally, a subclass can replace the methods and properties of the base class with its own new implementation - effectively overriding the original behavior and replacing it with new behaviors. Essentially inheritance is a way of merging functionality from an existing class into our new subclass. Inheritance also defines rules for how these methods, properties, and events can be merged.

What's an assembly?
Assemblies are the building blocks of .NET Framework applications; they form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions. An assembly is a collection of types and resources that are built to work together and form a logical unit of functionality. An assembly provides the common language runtime with the information it needs to be aware of type implementations. To the runtime, a type does not exist outside the context of an assembly.

Describe the difference between inline and code behind - which is best in a loosely coupled solution?
ASP.NET supports two modes of page development: Page logic code that is written inside

What is the difference between an EXE and a DLL?
An EXE can run independently, whereas DLL will run within an EXE. DLL is an in-process file and EXE is an out-process file

What is strong-typing versus weak-typing? Which is preferred? Why?
Strong type is checking the types of variables as soon as possible, usually at compile time. While weak typing is delaying checking the types of the system as late as possible, usually to run-time. Which is preferred depends on what you want. For scripts & quick stuff you’ll usually want weak typing, because you want to write as much less code as possible. In big programs, strong typing can reduce errors at compile time.

What are PDBs? Where must they be located for debugging to work?
Answer1:
To debug precompiled components such as business objects and code-behind modules, you need to generate debug symbols. To do this, compile the components with the debug flags by using either Visual Studio .NET or a command line compiler such as Csc.exe (for Microsoft Visual C# .NET) or Vbc.exe (for Microsoft Visual Basic .NET).

Using Visual Studio .NET
1. Open the ASP.NET Web Application project in Visual Studio .NET.
2. Right-click the project in the Solution Explorer and click Properties.
3. In the Properties dialog box, click the Configuration Properties folder.
4. In the left pane, select Build.
5. Set Generate Debugging Information to true.
6. Close the Properties dialog box.
7. Right-click the project and click Build to compile the project and generate symbols (.pdb files).

Answer2:
A program database (PDB) file holds debugging and project state information that allows incremental linking of a Debug configuration of your program.
The linker creates project.PDB, which contains debug information for the project’s EXE file. The project.PDB contains full debug information, including function prototypes, not just the type information found in VCx0.PDB. Both PDB files allow incremental updates.
They should be located at bin\Debug directory

What is cyclomatic complexity and why is it important?
Cyclomatic complexity is a computer science metric (measurement) developed by Thomas McCabe used to generally measure the complexity of a program. It directly measures the number of linearly independent paths through a program’s source code.

The concept, although not the method, is somewhat similar to that of general text complexity measured by the Flesch-Kincaid Readability Test.

Cyclomatic complexity is computed using a graph that describes the control flow of the program. The nodes of the graph correspond to the commands of a program. A directed edge connects two nodes, if the second command might be executed immediately after the first command. By definition,

CC = E - N + P

where
CC = cyclomatic complexity
E = the number of edges of the graph
N = the number of nodes of the graph
P = the number of connected components.

What is FullTrust? Do GAC’ed assemblies have FullTrust?
Your code is allowed to do anything in the framework, meaning that all (.Net) permissions are granted. The GAC has FullTrust because it’s on the local HD, and that has FullTrust by default, you can change that using caspol

What does this do? gacutil /l | find /i “about”
Answer1:
This command is used to install strong typed assembly in GAC

Answer2:
gacutil.exe is used to install strong typed assembly in GAC. gacutil.exe /l is used to lists the contents of the global assembly cache. |(pipe) symbol is used to filter the output with another command. find /i “about” is to find the text “about” on gacutil output. If any lines contains the text “about” then that line will get displayed on console window.

Contrast OOP and SOA. What are tenets of each
Service Oriented Architecture. In SOA you create an abstract layer that your applications use to access various “services” and can aggregate the services. These services could be databases, web services, message queues or other sources. The Service Layer provides a way to access these services that the applications do not need to know how the access is done. For example, to get a full customer record, I might need to get data from a SGL Server database, a web service and a message queue. The Service layer hides this from the calling application. All the application knows is that it asked for a full customer record. It doesn’t know what system or systems it came from or how it was retrieved.

How does the XmlSerializer work? What ACL permissions does a process using it require?
XmlSerializer requires write permission to the system’s TEMP directory.

Why is catch(Exception) almost always a bad idea?
Well, if at that point you know that an error has occurred, then why not write the proper code to handle that error instead of passing a new Exception object to the catch block? Throwing your own exceptions signifies some design flaws in the project.

What is the difference between Debug. Write and Trace. Write? When should each be used?
Answer1:
The Debug. Write call won’t be compiled when the DEBUG symbol is not defined (when doing a release build). Trace. Write calls will be compiled. Debug. Write is for information you want only in debug builds, Trace. Write is for when you want it in release build as well. And in any case, you should use something like log4net because that is both faster and better

Answer2:
Debug. Write & Trace. write - both works in Debug mode, while in Release Mode,Trace.write only will work .Try changing the Active Config property of Solution in Property page nd find the difference. Debug.write is used while debugging a project and Trace.write is used in Released version of Applications.

What is the difference between a Debug and Release build? Is there a significant speed difference? Why or why not?
Debug build contain debug symbols and can be debugged while release build doesn’t contain debug symbols, doesn’t have [Conational(”DEBUG”)] methods calls compiled, can’t be debugged (easily, that is), less checking, etc. There should be a speed difference, because of disabling debug methods, reducing code size etc but that is not a guarantee (at least not a significant one)

Contrast the use of an abstract base class against an interface?
Answer1:
In the interface all methods must be abstract, in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes

Answer2:
Whether to Choose VB.NET/C#.
Both the languages are using same classes and namespaces. Once it compile and generates MSIL, there is no meaning of which language it was written. If you are Java/C++ programmer better to choose C# for same coding style otherwise you can choose VB.net.

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What is the difference between a.Equals(b) and a == b?
Answer1:
a=b is used for assigning the values (rather then comparison) and a==b is for comparison.

Answer2:
a == b is used to compare the references of two objects
a.Equals(b) is used to compare two objects

Answer3:
A equals b -> copies contents of b to a
a == b -> checks if a is equal to b

Answer4:
Equals method compares both type and value of the variable, while == compares value.
int a = 0;
bool b = 0

if(a.Equals(b))

Answer5:
a.Equals(b) checks whether the Type of a is equal to b or not! Put it in another way,
Dim a As Integer = 1
Dim b As Single = 1

a.Equals(b) returns false. The Equals method returns a boolean value.
a == b is a simple assignment statement.

Answer6:
a.equals(b) will check whether the “b” has same type as “a” has and also has the same data as “a” has.
a==b will do the same thing.
if you have done this in c++ under “operator overloading” than you guys must be aware of this sytaxts. they are doing the same thing there is only sytaxtical difference.
let me explain it in different manner.
a==b : means compare “b” with “a”. always left hand side expression evaluated first so here in this case “a” (considered an object) will call the overloaded operator “=” which defines “Equals(object)” method in it’s class. thus, ultimately a.equals(b) goanna called.
so the answer is: both will perform the same task. they are different by syntaxt

Answer7:
Difference b/w a==b,a.Equals(b)
a.Equals(b):
The default implementation of Equals supports reference equality only, but derived classes can override this method to support value equality.

For reference types, equality is defined as object equality; that is, whether the references refer to the same object. For value types, equality is defined as bitwise equality
== :
For predefined value types, the equality operator (==) returns true if the values of its operands are equal, false otherwise. For reference types other than string, == returns true if its two operands refer to the same object. For the string type, == compares the values of the strings.

How would one do a deep copy in .NET?
Answer1:
System.Array.CopyTo() - Deep copies an Array

Answer2:
How would one do a deep copy in .NET?
The First Approach.
1.Create a new instance.
2.Copy the properties from source instance to newly created instance.
[Use reflection if you want to write a common method to achive this]

The Second Approach.
1. Serialize the object and deserialize the output.
: Use binary serialization if you want private variables to be copied.
: Use xml Serialization if you dont want private variable to be copied.

What is boxing?
Boxing is an implicit conversion of a value type to the type object
int i = 123; // A value type
Object box = i // Boxing
Unboxing is an explicit conversion from the type object to a value type
int i = 123; // A value type object box = i; // Boxing
int j = (int)box; // Unboxing

Is string a value type or a reference type?
Answer1:
String is Reference Type.
Value type - bool, byte, chat, decimal, double, enum , float, int, long, sbyte, short,strut, uint, ulong, ushort
Value types are stored in the Stack
Reference type - class, delegate, interface, object, string
Reference types are stored in the Heap


Answer2:
Yes String is reference type. C# gives two types of variable reference and value type. string and object are reference type.

How does the lifecycle of Windows services differ from Standard EXE?
Windows services lifecycle is managed by “Service Control Manager” which is responsible for starting and stopping the service and the applications do not have a user interface or produce any visual output, but “Standard executable” doesn’t require Control Manager and is directly related to the visual output

What’s wrong with a line like this? DateTime.Parse(myString)
the result returned by this function is not assigned to anything, should be something like varx = DateTime.Parse(myString)

NET is Compile Time OR RunTime Environment?
.Net’s framework has CLS,CTS and CLR.CTS checks declartion of types at the time when u write code and CLS defines some rules and restrictions.and CLR comile everything at runtime with following benefits: Vastly simplified development Seamless integration of code written in various languages Evidence-based security with code identity Assembly-based deployment that eliminates DLL Hell Side-by-side versioning of reusable components Code reuse through implementation inheritance Automatic object lifetime management Self describing objects

Describe the role of inetinfo.exe, aspnet_isapi.dll andaspnet_wp.exe in the page loading process.
inetinfo.exe is theMicrosoft IIS server running, handling ASP.NET requests among other things.When an ASP.NET request is received (usually a file with .aspx extension),the ISAPI filter aspnet_isapi.dll takes care of it by passing the request tothe actual worker process aspnet_wp.exe.

What’s the difference between Response.Write() andResponse.Output.Write()?
The latter one allows you to write formattedoutput.

What methods are fired during the page load?
Init() - when the pageis
instantiated, Load() - when the page is loaded into server memory,PreRender()
- the brief moment before the page is displayed to the user asHTML, Unload()
- when page finishes loading.

Where does the Web page belong in the .NET Framework class hierarchy?
System.Web.UI.Page

Where do you store the information about the user’s locale?
System.Web.UI.Page.Culture

What’s the difference between Codebehind="MyCode.aspx.cs" andSrc="MyCode.aspx.cs"?
CodeBehind is relevant to Visual Studio.NET only.

What’s a bubbled event?
When you have a complex control, like DataGrid, writing an event processing routine for each object (cell, button, row, etc.) is quite tedious. The controls can bubble up their event handlers, allowing the main DataGrid event handler to take care of its constituents.

Suppose you want a certain ASP.NET function executed on MouseOver overa certain button. Where do you add an event handler?
It’s the Attributesproperty,
the Add function inside that property. So

btnSubmit.Attributes.Add("onMouseOver","someClientCode();")

A simple”Javascript:ClientCode();” in the button control of the .aspx page will attach the handler (javascript function)to the onmouseover event.

What data type does the RangeValidator control support?
Integer,String and Date.



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