The.NET Framework (pronounced as "dot net") is a proprietarysoftware framework developed byMicrosoft that runs primarily onMicrosoft Windows. It was the predominant implementation of theCommon Language Infrastructure (CLI) until being superseded by the cross-platform.NET project. It includes a largeclass library calledFramework Class Library (FCL) and provideslanguage interoperability (each language can use code written in other languages) across severalprogramming languages. Programs written for .NET Framework execute in asoftware environment (in contrast to ahardware environment) named theCommon Language Runtime (CLR). The CLR is anapplication virtual machine that provides services such as security,memory management, andexception handling. As such, computer code written using .NET Framework is called "managed code". FCL and CLR together constitute the .NET Framework.

.NET Framework

.NET Framework component stack
Developer(s)	Microsoft
Initial release	February 13, 2002; 23 years ago (2002-02-13)
Stable release	4.8.1 / August 9, 2022; 2 years ago (2022-08-09)[1]
Operating system	Windows 98 or later,Windows NT 4.0 or later
Platform	IA-32,x86-64, andARM
Included with	Microsoft Windows
Successor	.NET
Type	Software framework
License	Mixed; see§ Licensing
Website	dotnet.microsoft.com

FCL provides theuser interface,data access,database connectivity,cryptography,web application development, numericalgorithms, andnetwork communications. Programmers produce software by combining theirsource code with the .NET Framework and other libraries. The framework is intended to be used by most new applications created for the Windows platform. Microsoft also produces anintegrated development environment for .NET software calledVisual Studio.

.NET Framework began asproprietary software, although the firm worked tostandardize the software stack almost immediately, even before its first release. Despite the standardization efforts, developers, mainly those in thefree and open-source software communities, expressed their unease with the selected terms and the prospects of any free and open-source implementation, especially regardingsoftware patents. Since then, Microsoft has changed .NET development to more closely follow a contemporary model of a community-developed software project, including issuing an update to its patent promising to address the concerns.^[2]

In April 2019, Microsoft released .NET Framework 4.8, the last major version of the framework as a proprietary offering, followed by .NET Framework 4.8.1 in August 2022. Only monthly security and reliabilitybug fixes to that version have been released since then. No further changes to that version are planned. The .NET Framework will continue to be included with future releases of Windows and continue to receive security updates, with no plans to remove it as of September 2024.^[3]

Microsoft began developing .NET Framework in the late 1990s, originally under the name of Next Generation Windows Services (NGWS), as part of the.NET strategy. By early 2000, the first beta versions of .NET 1.0 were released.

In August 2000,Microsoft andIntel worked to standardizeCommon Language Infrastructure (CLI) andC#. By December 2001, both were ratifiedby ECMA standards.^[4][5]International Organization for Standardization (ISO) followed in April 2003. The current version of ISO standards are ISO/IEC 23271:2012 and ISO/IEC 23270:2006.^[6][7]

While Microsoft and their partners hold patents for CLI and C#, ECMA and ISO require that all patents essential to implementation be made available under "reasonable and non-discriminatory terms". The firms agreed to meet these terms, and to make the patents available royalty-free. However, this did not apply to the part of the .NET Framework not covered by ECMA-ISO standards, which includedWindows Forms,ADO.NET, andASP.NET. Patents that Microsoft holds in these areas may have deterred non-Microsoft implementations of the full framework.^[8]

Windows Vista is the first client version of Windows that integrated the .NET Framework.

On October 3, 2007, Microsoft announced that thesource code for .NET Framework 3.5 libraries was to become available under theMicrosoft Reference Source License (Ms-RSL^[a]).^[9] The source code repository became available online on January 16, 2008, and included BCL, ASP.NET, ADO.NET, Windows Forms, WPF, and XML.Scott Guthrie of Microsoft promised that LINQ, WCF, and WF libraries were being added.^[10]

The.NET Compact Framework and.NET Micro Framework variants of the .NET Framework provided support for other Microsoft platforms such asWindows Mobile,Windows CE and other resource-constrained embedded devices.Silverlight provided support forweb browsers via plug-ins.

In November 2014, Microsoft also produced an update to its patent grants, which further extends the scope beyond its prior pledges. Prior projects likeMono existed in a legalgrey area because Microsoft's earlier grants applied only to the technology in "covered specifications", including strictly the 4th editions each of ECMA-334 and ECMA-335. The new patent promise, however, places no ceiling on the specification version, and even extends to any .NET runtime technologies documented on MSDN that have not been formally specified by the ECMA group, if a project chooses to implement them. This allows Mono and other projects to maintain feature parity with modern .NET features that have been introduced since the 4th edition was published without being at risk of patent litigation over the implementation of those features. The new grant does maintain the restriction that any implementation must maintain minimum compliance with the mandatory parts of the CLI specification.^[11]

On March 31, 2016, Microsoft announced atMicrosoft Build that they will completelyrelicense Mono under anMIT License even in scenarios where formerly a commercial license was needed.^[12] Microsoft also supplemented its prior patent promise for Mono, stating that they will not assert any "applicable patents" against parties that are "using, selling, offering for sale, importing, or distributing Mono."^[13][14] It was announced that the Mono Project was contributed to the .NET Foundation. These developments followed the acquisition ofXamarin, which began in February 2016 and was finished on March 18, 2016.^[15]

Microsoft's press release highlights that the cross-platform commitment now allows for a fully open-source, modern server-side .NET stack. Microsoft released the source code for WPF, Windows Forms andWinUI on December 4, 2018.^[16]

Common Language Infrastructure (CLI) provides a language-neutral platform for application development and execution. By implementing the core aspects of .NET Framework within the scope of CLI, these functions will not be tied to one language but will be available across the many languages supported by the framework.

.NET Framework includes the Common Language Runtime (CLR). It serves as the execution engine of .NET Framework and offers many services such asmemory management,type safety,exception handling,garbage collection, security andthread management. All programs written for .NET Framework are executed by the CLR.

Programs written for .NET Framework are compiled intoCommon Intermediate Language code (CIL), as opposed to being directly compiled intomachine code. During execution, an architecture-specificjust-in-time compiler (JIT) turns theCIL code into machine code.

Compiled CLI code is stored inCLI assemblies. As mandated by the specification, assemblies are stored inPortable Executable (PE) file format, common on Windows platform for alldynamic-link library (DLL) andexecutableEXE files. Each assembly consists of one or more files, one of which must contain a manifest bearing themetadata for the assembly. The complete name of an assembly (not to be confused with the file name on disk) contains its simple text name, version number, culture, andpublic key token. Assemblies are considered equivalent if they share the same complete name.

A private key can also be used by the creator of the assembly forstrong naming. The public key token determines the real-world identity of the assembly's signer. Only those knowing their private key (of the double-key cryptography system) can sign assemblies that have the same strong name as a prior version assembly. Strong naming is required to add assemblies toGlobal Assembly Cache.

Starting with Visual Studio 2015, .NET Native compilation technology allows for the compilation of .NET code ofUniversal Windows Platform apps directly to machine code rather than CIL code, but the app must be written in either C# or Visual Basic.NET.^[17]

.NET Framework includes an implementation of the CLI foundationalStandard Libraries. The .NET Framework Class Library (FCL) is organized in a hierarchy ofnamespaces. Most of the built-inapplication programming interfaces (APIs) are part of eitherSystem.* orMicrosoft.* namespaces. These class libraries implement many common functions, such as file reading and writing, graphic rendering, database interaction, and XML document manipulation. The class libraries are available for allCLI compliant languages. The FCL implements the CLIBase Class Library (BCL) and other class libraries—some are specified by CLI and other are Microsoft specific.

BCL includes a small subset of the entire class library and is the core set of classes that serve as the basicAPI of CLR.^[18] For .NET Framework most classes considered being part of BCL reside inmscorlib.dll,System.dll andSystem.Core.dll. BCL classes are available in .NET Framework as well as in CLI's alternative implementations including.NET Compact Framework,Microsoft Silverlight,.NET Core andMono.

FCL refers to the entire class library that ships with .NET Framework. It includes BCL, an expanded set of libraries, includingWindows Forms,ASP.NET, andWindows Presentation Foundation (WPF), and also extensions to the base class librariesADO.NET,Language Integrated Query (LINQ),Windows Communication Foundation (WCF), andWorkflow Foundation (WF). FCL is much larger in scope than standard libraries for languages likeC++, and comparable in scope tostandard libraries of Java.

With the introduction of alternative CLI's implementations (e.g., Silverlight), Microsoft introduced the concept of Portable Class Libraries (PCL) allowing a consuming library to run on more than one implementation. With the further proliferation of implementations, the PCL approach failed to scale (PCLs are defined intersections of API surface between two or more implementations).^[19] As the next evolutionary step of PCL, the .NET Standard Library was created retroactively based on theSystem.Runtime.dll based APIs found in UWP and Silverlight. New CLI's implementations are encouraged to implement a version of the Standard Library allowing them to run existent third-party libraries with no need of creating new versions of them. The .NET Standard Library allows an independent evolution of the library and app model layers within the .NET architecture.^[20]

NuGet is the package manager for all .NET platforms. It is used to retrieve third-party libraries into a .NET project with a global library feed at NuGet.org.^[21] Private feeds can be maintained separately, e.g., by a build server or a file system directory.

Microsoft introducedC++/CLI inVisual Studio 2005, which is a language and means of compilingVisual C++ programs to run within the .NET Framework. Some parts of the C++ program still run within an unmanagedVisual C++ Runtime, while specially modified parts are translated intoCIL code and run with the .NET Framework'sCLR.

Assemblies compiled using the C++/CLI compiler are termed mixed-mode assemblies since they contain native and managed code in the same DLL.^[22] Such assemblies are more complex to reverse engineer since .NETdecompilers such as.NET Reflector reveal only the managed code.

Because computer systems commonly require interaction between newer and older applications, .NET Framework provides means to access functions implemented in newer and older programs that execute outside .NET environment. Access toComponent Object Model (COM) components is provided inSystem.Runtime.InteropServices andSystem.EnterpriseServices namespaces of the framework. Access to other functions is viaPlatform Invocation Services (P/Invoke). Access to .NET functions from native applications is via the reverse P/Invoke function.

.NET Framework introduces aCommon Type System (CTS) that defines all possibledata types andprogramming constructs supported by CLR and how they may or may not interact conforming to CLI specifications. Because of this feature, .NET Framework supports the exchange of types and object instances between libraries and applications written usingany conforming CLI language.

CTS and the CLR used in .NET Framework also enforcetype safety. This prevents ill-defined casts, wrong method invocations, and memory size issues when accessing an object. This also makes most CLI languagesstatically typed (with or withouttype inference). However, starting with .NET Framework 4.0, theDynamic Language Runtime extended the CLR, allowing dynamically typed languages to be implemented atop the CLI.

While Microsoft has never implemented the full framework on any system except Microsoft Windows, it has engineered the framework to be cross-platform,^[23] and implementations are available for other operating systems (seeSilverlight and§ Alternative implementations). Microsoft submitted the specifications for CLI (which includes the Base Class Libraries, CTS, and CIL),^[24][25][26]C#,^[5] and C++/CLI^[27] to bothEcma International (ECMA) andInternational Organization for Standardization (ISO), making them available as official standards. This makes it possible for third parties to create compatible implementations of the framework and its languages on other platforms.

Core cross-platform .NET (formerly .NET Core) is officially available also for many Linux distributions and MacOS.^[28]

.NET Framework has its own security mechanism with two general features:Code Access Security (CAS), and validation and verification. CAS 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 Internet). CAS uses evidence to determine the permissions granted to the code. When calling code demands that it be granted a specific permission, CLR performs a call stack walk checking every assembly of each method in the call stack for the required permission; if any assembly is not granted the permission, it will throw a security exception.

ManagedCIL bytecode is easier toreverse-engineer than native code, unlessobfuscated.^[29] .NETdecompiler programs enable developers with no reverse-engineering skills to view the source code behind unobfuscated .NET assemblies. In contrast, apps compiled to native machine code are much harder to reverse-engineer, and source code is almost never produced successfully, mainly because of compiler optimizations and lack ofreflection.^[30] This creates concerns in the business community over the possible loss oftrade secrets and the bypassing of license control mechanisms. To mitigate this, Microsoft has includedDotfuscator Community Edition withVisual Studio .NET since 2002.^[b] Third-party obfuscation tools are also available from vendors such asVMware, V.i. Labs,Turbo, andRed Gate Software. Method-level encryption tools for .NET code are available from vendors such asSafeNet.

CLR frees the developer from the burden of managing memory (allocating and freeing up when done); it handles memory management itself by detecting when memory can be safely freed. Instantiations of .NET types (objects) are allocated from the managed heap; a pool of memory managed by CLR. As long as a reference to an object exists, which may be either direct, or via agraph of objects, the object is considered to be in use. When no reference to an object exists, and it cannot be reached or used, it becomes garbage, eligible for collection.

.NET Framework includes agarbage collector (GC) which runs periodically, on a separatethread from the application's thread, that enumerates all the unusable objects and reclaims the memory allocated to them. It is a non-deterministic, compacting,mark-and-sweep garbage collector. GC runs only when a set amount of memory has been used or there is enough pressure for memory on the system. Since it is not guaranteed when the conditions to reclaim memory are reached, GC runs arenon-deterministic. Each .NET application has a set of roots, which are pointers to objects on the managed heap (managed objects). These include references to static objects, objects defined as local variables or method parameters currently in scope, and objects referred to by CPU registers.^[31] When GC runs, it pauses the application and then, for each object referred to in the root, itrecursively enumerates all the objects reachable from the root objects and marks them as reachable. It uses CLI metadata andreflection to discover the objects encapsulated by an object, and then recursively walk them. It then enumerates all the objects on the heap (which were initially allocated contiguously) using reflection. All objects not marked as reachable are garbage.^[31] This is themark phase.^[32] Since the memory held by garbage is of no consequence, it is considered free space. However, this leaves chunks of free space between objects which were initially contiguous. The objects are thencompacted together to make free space on the managed heap contiguous again.^[31][32] Any reference to an object invalidated by moving the object is updated by GC to reflect the new location.^[32] The application is resumed after garbage collection ends. The latest version of .NET framework uses concurrent garbage collection along with user code, making pauses unnoticeable, because it is done in the background.^[33]

The garbage collector used by .NET Framework is alsogenerational.^[34] Objects are assigned ageneration. Newly created objects are taggedGeneration 0. Objects that survive one garbage collection are taggedGeneration 1. Generation 1 objects that survive another collection areGeneration 2. The framework uses up to Generation 2 objects.^[34] Higher generation objects are garbage collected less often than lower generation objects. This raises the efficiency of garbage collection, as older objects tend to have longer lifetimes than newer objects.^[34] By ignoring older objects in most collection runs, fewer checks and compaction operations are needed in total.^[34]

When an application is first launched, the .NET Framework compiles theCIL code into executable code using itsjust-in-time compiler, and caches the executable program into the .NET Native Image Cache.^[35][36] Due to caching, the application launches faster for subsequent launches, although the first launch is usually slower. To speed up the first launch, developers may use theNative Image Generator utility to manuallyahead-of-time compile and cache any .NET application.^[36]

The garbage collector, which is integrated into the environment, can introduce unanticipated delays of execution over which the developer has little direct control. "In large applications, the number of objects that the garbage collector needs to work with can become very large, which means it can take a very long time to visit and rearrange all of them."^[37]

.NET Framework provides support for callingStreaming SIMD Extensions (SSE) viamanaged code from April 2014 in Visual Studio 2013 Update 2. However,Mono has provided support forSIMD Extensions as of version 2.2 within the Mono.Simd namespace in 2009.^[38] Mono's lead developerMiguel de Icaza has expressed hope that this SIMD support will be adopted by CLR's ECMA standard.^[39] Streaming SIMD Extensions have been available inx86 CPUs since the introduction of thePentium III. Some other architectures such asARM andMIPS also have SIMD extensions. In case the CPU lacks support for those extensions, the instructions are simulated in software.^[40][41]

.NET Framework was the predominant implementation of CLI, until the release of.NET. Other implementations for parts of the framework exist. Although the runtime engine is described by an ECMA-ISO specification, other implementations of it may be encumbered bypatent issues; ISO standards may include the disclaimer, "Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights."^[42] It is harder to develop alternatives to FCL, which is not described by an open standard and may be subject to copyright restrictions. Also, parts of FCL have Windows-specific functions and behavior, so implementation on non-Windows platforms can be problematic.

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

• .NET Micro Framework is a .NET platform for extremely resource-constrained devices. It includes a small version of CLR and supports development inC# (though some developers were able to useVB.NET,^[43] albeit with an amount of hacking, and with limited functionalities) and debugging (in an emulator or on hardware), both usingMicrosoft Visual Studio. It also features a subset of .NET Framework Class Library (about 70 classes with about 420 methods), aGUI framework loosely based on WPF, and additional libraries specific to embedded applications.
• Mono is an implementation of CLI and FCL, and provides added functions. It is licensed asfree software under theMIT License. It includes support for ASP.NET, ADO.NET, and Windows Forms libraries for a wide range of architectures and operating systems. It also includes C# and VB.NET compilers.
• Portable.NET (part ofDotGNU) provides an implementation of CLI, parts of FCL, and a C# compiler. It supports a variety of CPUs and operating systems. The project was discontinued, with the last stable release in 2009.
• MicrosoftShared Source Common Language Infrastructure is anon-free implementation of CLR. However, the last version runs on Windows XP SP2 only, and has not been updated since 2006. Thus, it does not contain all features of version 2.0 of .NET Framework.
• CrossNet^[44] is an implementation of CLI and parts of FCL. It is free software using an open sourceMIT License.

Microsoft managed code frameworks and their components are licensed as follows:

• Microsoft Foundation Class Library (MFC), an object-oriented abstraction used to ease Windows-based application programming prior to .NET Framework
• .NET (formerly .NET Core)
• List of CLI languages
• Standard Libraries (CLI), the .NET standard libraries
• Base Class Library (BCL)

• Official website
• Overview of .NET Framework (MSDN)
• .NET Github repository