Faqs for .Net Framework

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:
The .NET Framework SDK is free and includes command-line compilers for C++, C#, and VB.NET and various other utilities to aid development.
SharpDevelop is a free IDE for C# and VB.NET.
Microsoft Visual Studio Express editions are cut-down versions of Visual Studio, for hobbyist or novice developers.There are different versions for C#, VB, web development etc. Originally the plan was to charge $49, but MS has decided to offer them as free downloads instead, at least until November 2006.
Microsoft Visual Studio Standard 2005 is around $300, or $200 for the upgrade.
Microsoft VIsual Studio Professional 2005 is around $800, or $550 for the upgrade.
At the top end of the price range are the Microsoft Visual Studio Team Edition for Software Developers 2005 with MSDN Premium and Team Suite editions.

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 the CTS, and how does it relate to the CLS?

CTS = Common Type System. This is the full range of types that the .NET runtime understands. Not all .NET languages support all the types in the CTS.

CLS = Common Language Specification. This is a subset of the CTS which all .NET languages are expected to support. The idea is that any program which uses CLS-compliant types can interoperate with any .NET program written in any language. This interop is very fine-grained - for example a VB.NET class can inherit from a C# class.
2.3 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.
2.4 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.
2.5 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.
2.6 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).
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?

Location and visibility: 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 normally stored in the global assembly cache, which is a repository of assemblies maintained by the .NET runtime. Shared assemblies are usually libraries of code which many applications will find useful, e.g. the .NET framework classes.
Versioning: The runtime enforces versioning constraints only on shared assemblies, not on private assemblies.

3.4 How do assemblies find each other?

By searching directory paths. There are several factors which can affect the path (such as the AppDomain host, and application configuration files), but for private assemblies the search path is normally the application's directory and its sub-directories. For shared assemblies, the search path is normally same as the private assembly path plus the shared assembly cache.
3.5 How does assembly versioning work?

Each assembly has a version number called the compatibility version. Also each reference to an assembly (from another assembly) includes both the name and version of the referenced assembly.

The version number has four numeric parts (e.g. 5.5.2.33). Assemblies with either of the first two parts different are normally viewed as incompatible. If the first two parts are the same, but the third is different, the assemblies are deemed as 'maybe compatible'. If only the fourth part is different, the assemblies are deemed compatible. However, this is just the default guideline - it is the version policy that decides to what extent these rules are enforced. The version policy can be specified via the application configuration file.

Remember: versioning is only applied to shared assemblies, not private assemblies.
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.