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The Microsoft .NET Framework is a software component that can be added to or is included with the Microsoft Windows operating system. It provides a large body of pre-coded solutions to common program requirements, and manages the execution of programs written specifically for the framework. The .NET Framework is a key Microsoft offering, and is intended to be used by most new applications created for the Windows platform.

The pre-coded solutions that form the framework's class library cover a large range of programming needs in areas including: user interface, data access, database connectivity, cryptography, web application development, numeric algorithms, and network communications. The functions of the class library are used by programmers who combine them with their own code to produce applications.

Programs written for the .NET Framework execute in a software environment that manages the program's runtime requirements. This runtime environment, which is also a part of the .NET Framework, is known as the Common Language Runtime (CLR). The CLR provides the appearance of an application virtual machine, so that programmers need not consider the capabilities of the specific CPU that will execute the program. The CLR also provides other important services such as security mechanisms, memory management, and exception handling. The class library and the CLR together compose the .NET Framework. The framework is intended to make it easier to develop computer applications and to reduce the vulnerability of applications and computers to security threats.

First released in 2002, it is included with Windows XP SP2, Windows Server 2003 and Windows Vista, and can be installed on most older versions of Windows.

Design goals and principal features

Microsoft .NET Framework was designed with several intentions:

  • Interoperability - Because interaction between new and older applications is commonly required, the .NET Framework provides means to access functionality that is implemented in programs that execute outside the .NET environment. Access to COM components is provided in the EnterpriseServices namespace of the framework, and access to other functionality is provided using the P/Invoke feature.
  • Common Runtime Engine - Programming languages on the .NET Framework compile into an intermediate language known as the Common Intermediate Language, or CIL (formerly known as Microsoft Intermediate Language, or MSIL). In Microsoft's implementation, this intermediate language is not interpreted, but rather compiled in a manner known as just-in-time compilation (JIT) into native code. The combination of these concepts is called the Common Language Infrastructure (CLI), a specification; Microsoft's implementation of the CLI is known as the Common Language Runtime (CLR).
  • Language Independence - The .NET Framework introduces a Common Type System, or CTS. The CTS specification defines all possible datatypes and programming constructs supported by the CLR and how they may or may not interact with each other. Because of this feature, the .NET Framework supports development in multiple programming languages. This is discussed in more detail in the .NET languages section below.
  • Base Class Library - The Base Class Library (BCL), sometimes referred to as the Framework Class Library (FCL), is a library of types available to all languages using the .NET Framework. The BCL provides classes which encapsulate a number of common functions, including file reading and writing, graphic rendering, database interaction and XML document manipulation.
  • Simplified Deployment - Installation of computer software must be carefully managed to ensure that it does not interfere with previously installed software, and that it conforms to increasingly stringent security requirements. The .NET framework includes design features and tools that help address these requirements.
  • Security - .NET allows for code to be run with different trust levels without the use of a separate sandbox.

A design goal of the .NET Framework was to support platform independence. That is, a program written to use the framework should run without change on any type of computer for which the framework is implemented. However, Microsoft has only implemented the full .NET framework on the Windows operating system. Microsoft and others have implemented portions of the framework on non-Windows systems, but to date these implementations are neither complete nor widely used, so cross-platform support has not been achieved.

.NET Framework architecture

Common Language Infrastructure (CLI)

The most important component of the .NET Framework lies within the Common Language Infrastructure, or CLI. The purpose of the CLI is to provide a language-agnostic platform for application development and execution, including, but not limited to, components for exception handling, garbage collection, security, and interoperability. Microsoft's implementation of the CLI is called the Common Language Runtime, or CLR. The CLR is composed of four primary parts:

  • Common Type System (CTS)
  • Common Language Specification (CLS)
  • Just-In-Time Compiler (JIT)
  • Virtual Execution System (VES)


The intermediate CIL code is housed in .NET assemblies, which for the Windows implementation means a Portable Executable (PE) file (EXE or DLL). Assemblies are the .NET unit of deployment, versioning and security. The assembly consists of one or more files, but one of these must contain the manifest, which has the metadata for the assembly. The complete name of an assembly contains its simple text name, version number, culture and public key token; it must contain the name, but the others are optional. The public key token is generated when the assembly is created, and is a value that uniquely represents the name and contents of all the assembly files, and a private key known only to the creator of the assembly. Two assemblies with the same public key token are guaranteed to be identical. If an assembly is tampered with (for example, by hackers), the public key can be used to detect the tampering.


All CIL is Self-Describing through .NET metadata. The CLR checks on metadata to ensure that the correct method is called. Metadata is usually generated by language compilers but developers can create their own metadata through custom attributes. Metadata also contain all the information about assembly.

Base Class Library (BCL)

The Base Class Library (BCL), sometimes incorrectly referred to as the Framework Class Library (FCL) (which is a superset including the Microsoft.* namespaces), is a library of classes available to all languages using the .NET Framework. The BCL provides classes which encapsulate a number of common functions such as file reading and writing, graphic rendering, database interaction, XML document manipulation, and so forth. The BCL is much larger than other libraries, but has much more functionality in one package.


.NET has its own security mechanism, with two general features: Code Access Security (CAS), and validation and verification. Code Access Security is based on evidence that is associated with a specific assembly. Typically the evidence is the source of the assembly (whether it is installed on the local machine, or has been downloaded from the intranet or Internet). Code Access Security uses evidence to determine the permissions granted to the code. Other code can demand that calling code is granted a specified permission. The demand causes the CLR to perform a call stack walk: every assembly of each method in the call stack is checked for the required permission and if any assembly is not granted the permission then a security exception is thrown.

When an assembly is loaded the CLR performs various tests. Two such tests are validation and verification. During validation the CLR checks that the assembly contains valid metadata and CIL, and it checks that the internal tables are correct. Verification is not so exact. The verification mechanism checks to see if the code does anything that is 'unsafe'. The algorithm used is quite conservative and hence sometimes code that is 'safe' is not verified. Unsafe code will only be executed if the assembly has the 'skip verification' permission, which generally means code that is installed on the local machine.

Standardization and licensing

In August 2000, Microsoft, HP, and Intel worked to standardize CLI and the C# programming language. By December 2001, both were ratified ECMA standards (ECMA 335 and ECMA 334). ISO followed in April 2003 (ISO/IEC 23271 and ISO/IEC 23270).

While Microsoft and their partners hold patents for CLI and C#, ECMA and ISO requires that all patents essential to implementation be made available under "reasonable and non-discriminatory (RAND) terms." In addition to meeting these terms, the companies have agreed to make the patents available royalty-free.

However, this does not apply for the part of the .NET Framework which is not covered by the ECMA/ISO standard, which includes Windows Forms, ADO.NET, and ASP.NET. Patents that Microsoft holds in these areas may deter non-Microsoft implementations of the full framework.


Microsoft started development on the .NET Framework in the late 90s originally under the name of Next Generation Windows Services (NGWS). By late 2000 the first beta versions of .NET 1.0 were being released.

Version NameVersion NumberRelease Date
1.0 Beta 11.0.????.02000-11
1.0 Beta 21.0.2914.02001-06-20
1.0 RTM1.0.3705.02002-01-05
1.0 SP11.0.3705.2092002-03-19
1.0 SP21.0.3705.2882002-08-07
1.0 SP31.0.3705.60182004-08-31
1.1 RTM1.1.4322.5732003-04-01
1.1 SP11.1.4322.20322004-08-30
1.1 SP1 (Windows Server 2003 Version)1.1.4322.23002005-03-30
2.0 RTM2.0.50727.422005-11-07
3.0 RTM3.0.4506.302006-11-06
3.5 Beta 13.52007-04-27
3.5 Beta 23.52007-07-26

.NET Framework 1.0

This is the first release of the .NET Framework that was released on February 13, 2002. It is available on its own as a redistributable package or in a software development kit. It is also part of the first release of Microsoft Visual Studio .NET (also known as Visual Studio .NET 2002).

.NET Framework 1.1

This is the first major .NET Framework upgrade. It is available on its own as a redistributable package or in a software development kit, and was published April 3, 2003. It is also part of the second release of Microsoft Visual Studio .NET (released as Visual Studio .NET 2003). This is the first version of the .NET Framework to be included as part of the Windows operating system - it is part of Windows Server 2003. Windows Server 2003 originally shipped with the 1.1 RTM version.

Changes since 1.0

  • Built-in support for mobile ASP.NET controls. Previously available as an add-on for .NET Framework, now part of the framework.
  • Security changes - enable Windows Forms assemblies to execute in a semi-trusted manner from the Internet, and enable Code Access Security in ASP.NET applications.
  • Built-in support for ODBC and Oracle databases. Previously available as an add-on for .NET Framework 1.0, now part of the framework.
  • .NET Compact Framework - a version of the .NET Framework for small devices.
  • Internet Protocol version 6 (IPv6) support.
  • Numerous API changes.

.NET Framework 2.0

Released with Visual Studio .NET 2005, Microsoft SQL Server 2005 and BizTalk 2006.

.NET Framework 2.0 shipped with Windows Server 2003 R2 (not installed by default).

Changes since 1.1

  • Numerous API changes.
  • A new hosting API for native applications wishing to host an instance of the .NET runtime. The new API gives a fine grain control on the behavior of the runtime with regards to multithreading, memory allocation, assembly loading and more (detailed reference). It was initially developed to efficiently host the runtime in Microsoft SQL Server, which implements its own scheduler and memory manager.
  • Full 64-bit support for both the x64 and the IA64 hardware platforms.
  • Language support for Generics built directly into the .NET CLR.
  • Many additional and improved ASP.NET web controls.
  • New data controls with declarative data binding.
  • New personalization features for ASP.NET, such as support for themes, skins and webparts.
  • .NET Micro Framework - a version of the .NET Framework related to the Smart Personal Objects Technology initiative more....

.NET Framework 3.0

.NET Framework 3.0, formerly called WinFX,[1] includes a new set of managed code APIs that are an integral part of Windows Vista and Windows Server 2008 operating systems. It is also available for Windows XP SP2 and Windows Server 2003 as a download. There are no major architectural changes included with this release; .NET Framework 3.0 includes version 2.0 of the Common Language Runtime.[2]

.NET Framework 3.0 consists of four major new components:

.NET Framework 3.5

In an interview with Channel 9, Jason Zander, general manager of the .NET Framework team at Microsoft, discussed version 3.5 of the framework.[3] This version will include a new compiler that will support new features such as Language Integrated Query (LINQ), as well as new language features in C# and VB.NET. This version of the framework, containing version 3.0 of the CLR (as opposed to CLR 2.0 in .NET Framework 3.0), will be included in Visual Studio 2008.

Beta 1 of .NET Framework was released on April 27, 2007.[4] This release includes the C# 3.0 and VB.NET 9.0 compilers, which add support for LINQ queries, extension methods, and anonymous types with static type inference. C# 3.0 also adds support for expression trees and lambda methods. The BCL includes classes for LINQ(along with its various providers including LINQ to Objects, LINQ to XML and LINQ to SQL), paging support for ADO.NET, ADO.NET synchronization API to synchronize local caches and server side datastores, asynchronous network I/O API among others.[5] It also includes a peer-to-peer networking stack, including a managed PNRP resolver,[6] and managed wrappers for WMI and Active Directory APIs.[7] It also includes enhancements to the WCF and WF runtimes that let WCF work with POX and JSON data, as well as expose WF workflows as WCF services.[8] .NET Framework 3.5 also includes support for HTTP pipelining and syndication feeds.[8]

Future development

Microsoft has not yet made public a roadmap of the development plans for future edition of .NET framework, but has released general information regarding it. Future revisions will have better support for parallel programs, that target multi-core or distributed systems.[9] Technologies like PLINQ, a parallel implementation of the LINQ engine, and Concurrency and Coordination Runtime can make an appearance.

.NET vs. Java and Java EE

The CLI and C# have many similarities to Sun's JVM and Java. They are strong competitors. Both are based on a virtual machine model that hides the details of the computer hardware on which their programs run. Both use their own intermediate byte-code, Microsoft calling theirs Common Intermediate Language (CIL; formerly MSIL) and Sun Java bytecode. On .NET, the byte-code is always compiled JIT; with Java the byte-code is either interpreted, compiled in advance, or compiled JIT. Both provide extensive class libraries that address many common programming requirements, and both address many security issues that are present in other approaches. The namespaces provided in the .NET Framework closely resemble the platform packages in Java EE API Specification both in style and invocation.

.NET in its complete form (Microsoft's implementation) is currently only fully available on Windows platforms,[10][11][12] whereas Java is fully available on many platforms.[13] .NET was built from the ground-up to support multiple programming languages while targeting Microsoft Windows; the Java platform was initially built to support only the Java language on many operating system platforms under the slogan, "Write once, run anywhere." Support for many programming languages has been added to the Java platform[14].

Microsoft's reference implementation of .NET is closed source, whereas Sun's reference implementation of Java is becoming open source (including the class library, the compiler, the virtual machine, and the various tools associated with the Java Platform).[15]

However, the third-party Mono project is developing an open source implementation of subsets of the .NET Framework, including the Common Language Runtime, for the Linux, Solaris, Mac OS, and Windows platforms. The current version supports version 1.1 of .NET,[16] and the project is planning full support for 2.0. Meanwhile, the Java Virtual Machine environment is host to many alternative languages not supported by Sun.


Some concerns and criticisms relating to .NET include:

  • With the introduction of the .NET framework, the old Visual Basic language was replaced by the Visual Basic .NET language, which caused controversy among transitioning developers.
  • Succeeding versions of .NET lock out older operating systems. For example, .NET v.3.0 and above excludes Windows 2000 and prior versions, as well as XP SP1[17]. This can be construed as a way by Microsoft to move away from previous versions, and force operating system purchases.
  • Although it is more a business decision than a real criticism on the framework, some people have expressed concern that the framework is too much tied to the Microsoft Windows operating system.[18] However, the existence of alternative implementations for other platforms (though not yet complete) begins to alleviate this criticism.
  • Several backward and forward incompatibilities exist between .NET 1.0, .NET 1.1, and .NET 2.0. These are well-documented however, and mostly include security fixes,[19] changes in the underlying implementation (such as the GetHashCode() implementation),[20] as well as marking many methods as Obsolete.[21] Additionally, the framework allows running different versions side-by-side, to alleviate problems of version incompatibility.[22]
  • Applications running in a managed environment such as the Microsoft framework's CLR or Java's JVM tend to require more system resources than functionally similar applications that access machine resources more directly. However, some applications have been shown to perform better in .NET than in their native version. This could be due to the use of relatively well-performing functions in the .NET framework, JITting of managed code, or other aspects of the CLR.[23][24]
  • As JIT languages can be more easily reverse-engineered than native code to algorithms used by an application, there is concern over possible loss of trade secrets and the bypassing of license control mechanisms. However, many obfuscation techniques already developed can help to prevent this; indeed Microsoft's Visual Studio 2005 includes such a tool produced by PreEmptive Solutions (see dotfuscator).
  • Microsoft has applied for patents on some parts of the Framеwork.[25] An agreement was made however between Microsoft and Novell whereby Microsoft agreed to not sue Novell or its customers for patent infringement, allowing the implementation of these parts in the open-source Mono implementation of .NET.[26][27] According to a statement on the blog of Mono project leader Miguel de Icaza,[28] this agreement extends to Mono, but only for Novell developers and users. It was criticized by the Open source community because it violates the principles of giving equal rights to all users of a particular program (see Patent Agreement with Microsoft). In February 2007, rumors circulated that the Free Software Foundation is reviewing Novell's right to sell GNU software, which makes up much of a Linux operating system, because of this agreement.[29] However Eben Moglen later said that he was quoted out of context,[30] and referring to possible changes that could be made to the GPL version 3, that would block similar deals in the future.

Alternative implementations

The Microsoft .NET Framework is the predominant implementation of .NET technologies. Other implementations for parts of the framework exist. Since the runtime engine is described by a ECMA/ISO specification, other implementations of it are unencumbered by copyright issues. It is more difficult to develop alternatives to the base class library (BCL), which is not described by an open standard, and may be subject to copyright restrictions. Additionally, parts of the BCL have Windows-specific functionality and behavior, so implementation on non-Windows platforms can be problematic.

Some alternative implementations of parts of the framework are listed here.

  • Microsoft's Shared Source Common Language Infrastructure is a shared source implementation of the CLR component of the .NET Framework. It runs on Microsoft Windows XP, FreeBSD, and Mac OS X 10.2.
  • Portable.NET (part of DotGNU) provides an implementation of the Common Language Infrastructure (CLI), portions of the .NET Base Class Library (BCL), and a C# compiler. It supports a variety of CPUs and operating systems.
  • Mono is an implementation of the CLI and portions of the .NET Base Class Library (BCL), and provides additional functionality. It is dual-licensed under free software and proprietary software licenses. Mono is being developed by Novell, Inc. It includes support for ASP.NET, ADO.NET, and evolving support for Windows Forms libraries. It also includes a C# compiler, and a VB.NET compiler is in pre-beta form.

See also

Components and libraries


  1. WinFX name change announcement
  2. ".NET Framework 3.0 Versioning and Deployment Q&A". Retrieved 2007-03-21. 
  3. Jason Zander on the WinFX to .NET FX 3.0 rename Streaming WMV file
  4. "Microsoft Pre-Release Software Microsoft .NET Framework 3.5 – Beta 1". Microsoft. April 27, 2007. Retrieved 2007-04-30. 
  5. "C# Online.NET Official Weblog". Retrieved 2007-06-24. 
  6. Kevin Hoffman. "Orcas' Hidden Gem - The managed PNRP stack". Retrieved 2007-06-24. 
  7. ".NET Framework 3.5". Retrieved 2007-06-24. 
  8. 8.0 8.1 Matt Winkle. "WCF and WF in "Orcas"". Retrieved 2007-06-14. 
  9. S. Somasegar. "The world of multi and many cores". Retrieved 2007-05-11. 
  10. .Net Framework 1.0 Redistributable Requirements MSDN, Accessed April 22, 2007
  11. .Net Framework 1.1 Redistributable Requirements MSDN, Accessed April 22, 2007
  12. .Net Framework 2.0 Redistributable Requirements MSDN, Accessed April 22, 2007
  13. JRE 5.0 Installation Notes Sun Developer Network, Access April 22, 2007
  14. [ Languages for the Java VM. Accessed May 11, 2007
  15. Free and Open Source Java Java Open Source centre. Accessed April 22, 2007
  16. "FAQ: General". Mono. 2006-12-20. Retrieved 2007-01-21. 
  17. .NET 3.0 System requirements
  18. see for example Bjarne Stroustrup's homepage
  19. Backwards Breaking Changes from version 1.1 to 2.0
  21. Compatibility Considerations and Version Changes
  22. Side-by-Side Execution of the .NET Framework
  23. The PHP Language Compiler for the .NET Framework
  24. IronPython: A fast Python implementation for .NET and Mono
  25. .Net patent could stifle standards effort
  26. Microsoft and Novell Announce Broad Collaboration on Windows and Linux Interoperability and Support>
  27. Mono FAQ: Licensing (Patents)
  28. "Statement on the blog of Mono project leader Miguel de Icaza regarding patent protection for Mono". 2006-11-04. Retrieved 2006-11-06. 
  29. Novell could be banned from selling Linux: group
  30. The Free Software Foundation vs. Novell?

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