.NET Framework

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The .NET Framework (pronounced "dot net") is a bundle of interdependent programs required to create and run programs using C# and multiple other programming languages on most versions of Microsoft Windows. Microsoft launched the framework in 2000, and since then, it has been standardized and widely adopted as a platform by many different programming languages. It has also been enabled for execution on the Linux operating systems, in addition to most versions of Microsoft Windows.

As of late 2012, version 4.5 of the .NET Framework has been released at the same time as the new Windows 8 operating system. .NET executable programs may run as applications on a desktop or on a web server as part of a web application. The .NET Framework is also available in a reduced version for hand-held devices such as smart phones.

How the platform works

The .NET Framework's centerpiece is an execution engine, also known as a virtual machine and sometimes called the common language runtime (CLR), that has been implemented for most versions of Microsoft Windows operating systems. When downloading updates for Microsoft Windows, the execution engine is identified as the .NET Framework, although it is only a portion of the entire framework. The .NET Framework component available through Windows Update also includes the thousands of pre-written library classes. This .NET runtime environment, including the class libraries, makes all Windows computer systems look alike so that programs written in the C# programming language, and several other programming languages that target the .NET Framework, can run identically on all of them.

The .NET Framework runtime environment hides the underlying operating system from the programmer. The platform consists of multiple programs, each of which provides a distinct portion of its overall capabilities. The essential components in the platform are the C# source code compiler, the .NET class libraries, and the .NET runtime environment. The C# source code compiler converts C# source code (files with an extension of .cs) into intermediate code (files with an extension of .exe). The runtime environment is a special program that takes as input .NET common language runtime programs (special .exe files) for execution according to the rules laid out in the .NET Framework specification.

The .NET Framework does not necessarily include an integrated development environment (IDE). Microsoft produces and sells the Microsoft Visual Studio IDE, and provides free, reduced versions of it for learners. Several other free IDE's exist.

Common Intermediate Language and JIT compilation

C# programs initially compile down to Common Intermediate Language (CIL) code; these programs are loaded and run by .NET common language runtime, or virtual machine. The .NET runtime is sophisticated; it implements the .NET intermediate language specification by means of a just-in-time (JIT) compiler that, at runtime, converts CIL code into native code. JIT compilation occurs on a per method basis only the first time each method is called, after which the native code for that method remains cached in memory; this means that subsequent executions of that method run as fast as native programs. .NET programs tend to "warm up" after they have been executing for awhile, and in some cases they execute as efficiently as native code after an initial startup period.

The .NET Framework also includes extensive libraries (pre-compiled into intermediate language) containing reusable code, as well as numerous ways for .NET applications to be deployed, including on a desktop machine for a single user, on a web server as a handler (returning "dynamic" information to web browsers), or on a web server as a web service endpoint (procedure callable across a network from any kind of platform). There are several other components and utilities as well.

Associated file extensions

Compiled .NET programs have an extension of .exe or .dll but differ in internal format from .exe or .dll files containing Intel native code. The .NET versions of such files are called assemblies rather than executables. .NET assemblies may contain, besides .NET intermediate code, encapsulated resources such as images. When people say component or library, they usually mean a .dll file, which is a dynamically linked library that does not run standalone; it only runs if called from within a running .exe program. A summary of the .NET file extensions is shown here:

Extension Contents Name Comments
.cs C# source code plain text file many other source languages possible
.exe metadata and intermediate code .NET assembly (standalone program) binary file in PE format, containing CLR code
.dll metadata and intermediate code .NET assembly (dynamically linked library) binary file in PE format, containing CLR code

Release History

Microsoft started development on the .NET Framework in the late 90s and, by late 2000, the first beta versions of .NET were being released. In the following table, RTM is omitted although it sometimes appears on official releases (it stands for Release to Manufacturing):

Version Name Version Number Release Date
1.0 Beta 1 1.0.????.0 Nov. 2000 Microsoft Integrated Development Environment (IDE)
1.0 Beta 2 1.0.2914.0 Jun. 2001
1.0 1.0.3705.0 Jan. 2002 Visual Studio .NET v1.0
1.0 SP1 1.0.3705.209 Mar. 2002
1.0 SP2 1.0.3705.288 Aug. 2002
1.0 SP3 1.0.3705.6018 Aug. 2004
1.1 1.1.4322.573 Apr. 2003 Visual Studio .NET v1.1
1.1 SP1 1.1.4322.2032 Aug. 2004
1.1 SP1 for Windows Server 2003 1.1.4322.2300 Mar. 2005
2.0 2.0.50727.42 Nov. 2005 Visual Studio 2005
3.0 3.0.4506.30 Nov. 2006
3.5 Beta 2 3.5.20706.1 Jul. 2007 Visual Studio .NET 2008 Beta 2 (code name Orcas)

Microsoft Visual Studio .NET and the Windows Designer

With each major release of the .NET Framework, Microsoft has also released a version of its integrated development environment (IDE), Visual Studio .NET. Visual Studio .NET is intended for developers who write programs in the C# or Visual Basic .NET programming languages. The popularity of both languages for Windows application programming has been increased by the presence of the Windows Designer in Visual Studio .NET. The Windows Designer allows users to drag and drop components such as text boxes and labels onto a window (called a form in Microsoft terminology). The Designer is really a code generator; it generates code to declare and instantiate components as a result of programmers dragging and dropping a visual representation of a component onto a form.

The Windows Designer was patterned after a similar feature in the Visual Basic IDE in the early 1990's; the Designer was adopted by the C# IDE due to its enormous popularity as a design tool for Visual Basic. There is an equivalent graphical "Designer" feature in Visual Studio .NET for developing ASP.NET web forms. When the .NET Framework first appeared, the Windows Designer was instrumental in gaining acceptance for the .NET Framework and the C# and VB.NET languages from the computer industry because of its helpfulness in rapid application development.


The virtual machine specification for the .NET Framework has been standardized by the ECMA International (ECMA), and based on that standard, the open-source Mono[1] project has implemented a .NET runtime engine for Linux. An important part of the ECMA standard is the Common Language Specification, or CLS, consisting of the specifications which programmers must follow in order to assure that code they write will interwork with other languages. There are 41 rules in the CLS specification, including just for example, that library components must not use case-sensitive symbols (since some programming languages are not case sensitive, like Visual Basic). Components which comply with all rules of the CLS can be marked with a meta-tag claiming CLS-compliance, and can thus be treated as such by the runtime engines and by compilers and other utilities. The ECMA standard was also adopted in 2006 by the International Standards Organization (ISO).

Competition with the Java platform

See a Comparison_of_Java_and_.NET.

Languages that target the .NET Framework

See the List of languages using the .NET Framework.


  1. "What is Mono?". Mono open sourc project. Retrieved on 2007-04-02.