Microsoft began developing .NET Framework in the late 1990s, originally under the name of Next Generation Windows Services (NGWS). By late 2000, the first beta versions of .NET 1.0 were released.
In August 2000, Microsoft, Hewlett-Packard, and Intel worked to standardize Common Language Infrastructure (CLI) and C#. By December 2001, both were ratified Ecma International (ECMA) standards. International Organisation for Standardisation (ISO) followed in April 2003. The current version of ISO standards are ISO-IEC 23271:2012 and ISO/IEC 23270:2006.
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 for the part of .NET Framework not covered by ECMA-ISO standards, which included Windows Forms, ADO.NET, and ASP.NET. Patents that Microsoft holds in these areas may have deterred non-Microsoft implementations of the full framework.
On 3 October 2007, Microsoft announced that the source code for .NET Framework 3.5 libraries was to become available under the Microsoft Reference Source License (Ms-RSL). The source code repository became available online on 16 January 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.
On 12 November 2014, Microsoft announced .NET Core, in an effort to include cross-platform support for .NET, the source release of Microsoft's CoreCLR implementation, source for the "entire […] library stack" for .NET Core, and the adoption of a conventional ("bazaar"-like) open-source development model under the consolation stewardship of the .NET Foundation. Miguel de Icaza describes .NET Core as a "redesigned version of .NET that is based on the simplified version of the class libraries", and Microsoft's Immo Landwerth explained that .NET Core would be "the foundation of all future .NET platforms". At the time of the announcement, the initial release of the .NET Core project had been seeded with a subset of the libraries' source code and coincided with the relicensing of Microsoft's existing .NET reference source away from the restrictions of the Ms-RSL. Landwerth acknowledged the disadvantages of the formerly selected shared source license, explaining that it made codename Rotor "a non-starter" as a community-developed open source project because it did not meet the criteria of an Open Source Initiative (OSI) approved license.
In November 2014, Microsoft also produced an update to its patent grants, which further extends the scope beyond its prior pledges. Prior projects like Mono existed in a legal grey 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.
On 31 March 2016, Microsoft announced at Microsoft Build that they will completely relicense Mono under an MIT License even in scenarios where formerly a commercial license was needed. Microsoft also supplemented its prior patent promise for Mono, stating that they won't assert any "applicable patents" against parties that are "using, selling, offering for sale, importing, or distributing Mono." It was announced that the Mono Project was contributed to the .NET Foundation. These developments followed the prior acquisition of Xamarin, which began in February 2016 and was finished on 18 March 2016.
Microsoft's press release highlights that the cross-platform commitment now allows for a fully open-source, modern server-side .NET stack. However, Microsoft does not plan to release the source for WPF or Windows Forms.
Common Language Infrastructure (CLI) provides a language-neutral platform for application development and execution, including functions for exception handling, garbage collection, security, and interoperability. 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. Microsoft's implementation of CLI is Common Language Runtime (CLR). It serves as the execution engine of .NET Framework. All .NET programs execute under the supervision of CLR, guaranteeing many properties and behaviors in the areas of memory management, security, and exception handling.
For computer programs to run on CLI, they need to be compiled into Common Intermediate Language (CIL) – as opposed to being compiled into machine code. Upon execution, an architecture-specific just-in-time compiler (JIT) turns the CIL code into machine code. To improve performance, however, .NET Framework comes with Native Image Generator (NGEN), which performs ahead-of-time compilation.
Compiled CIL code is stored in CLI assemblies. As mandated by the specification, assemblies are stored in Portable Executable (PE) file format, common on Windows platform for all dynamic-link library (DLL) and executable EXE files. Each assembly consists of one or more files, one of which must contain a manifest bearing the metadata 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, and public 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 for strong naming. The public key token identifies which private key an assembly is signed with. Only the creator of the keypair (typically .NET developer signing the assembly) can sign assemblies that have the same strong name as a prior version assembly, since the creator possesses the private key. Strong naming is required to add assemblies to Global Assembly Cache.
.NET Framework includes a set of standard class libraries. The class library is organized in a hierarchy of namespaces. Most of the built-in application programming interfaces (APIs) are part of either
Microsoft.* namespaces. These class libraries implement very many common functions, such as file reading and writing, graphic rendering, database interaction, and XML document manipulation. The class libraries are available for all CLI compliant languages. The class library is divided into two parts (with no clear boundary): Base Class Library (BCL) and Framework Class Library (FCL).
BCL includes a small subset of the entire class library and is the core set of classes that serve as the basic API of CLR. For .NET Framework most classes considered being part of BCL reside in
System.core.dll. BCL classes are available in .NET Framework as well as its alternative implementations including .NET Compact Framework, Microsoft Silverlight, .NET Core and Mono.
FCL is a superset of BCL and refers to the entire class library that ships with .NET Framework. It includes an expanded set of libraries, including the Windows Forms, ASP.NET, and Windows Presentation Foundation (WPF) but also extensions to the base class libraries ADO.NET, Language Integrated Query (LINQ), Windows Communication Foundation (WCF), and Workflow Foundation (WF). FCL is much larger in scope than standard libraries for languages like C++, and comparable in scope to standard libraries of Java.
With the introduction of alternative implementations (e.g., Silverlight), Microsoft introduced the concept of Portable Class Libraries (PCL) allowing a consuming library to run on more than one platform. With the further proliferation of .NET platforms, the PCL approach failed to scale (PCLs are defined intersections of API surface between two or more platforms). As the next evolutionary step of PCL, the .NET Standard Library was created retroactively based on the
System.Runtime.dll based APIs found in UWP and Silverlight. New .NET platforms are encouraged to implement a version of the standard library allowing them to re-use extant third-party libraries to run without new versions of them. The .NET Standard Library allows an independent evolution of the library and app model layers within the .NET architecture.
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. Private feeds can be maintained separately, e.g., by a build server or a file system directory.
Atop the class libraries, multiple app models are used to create applications. .NET Framework supports Console, Windows Forms, Windows Presentation Foundation, ASP.NET and ASP.NET Core applications by default. Other app models are offered by alternative implementations of the .NET Framework. Console, UWP and ASP.NET Core are available on .NET Core. Mono is used to power Xamarin app models for Android, iOS, and macOS. The retroactive architectural definition of app models showed up in early 2015 and was also applied to prior technologies like Windows Forms or WPF.
Microsoft introduced C++/CLI in Visual Studio 2005, which is a language and means of compiling Visual C++ programs to run within the .NET Framework. Some parts of the C++ program still run within an unmanaged Visual C++ Runtime, while specially modified parts are translated into CIL code and run with the .NET Framework's CLR.
Assemblies compiled using the C++/CLI compiler are termed mixed-mode assemblies, since they contain native and managed code in the same DLL. Such assemblies are also difficult to reverse engineer, since .NET decompilers 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 to Component Object Model (COM) components is provided in
System.EnterpriseServices namespaces of the framework. Access to other functions is via Platform Invocation Services (P/Invoke). Access to .NET functions from native applications is via reverse P/Invoke function.
.NET Framework introduces a Common Type System (CTS) that defines all possible data types and programming constructs supported by CLR and how they may or may not interact with each other conforming to CLI specification. Because of this feature, .NET Framework supports the exchange of types and object instances between libraries and applications written using any conforming .NET language.
CTS and the CLR used in .NET Framework also enforce type safety. This prevents ill-defined casts, wrong method invocations, and memory size issues when accessing an object. This also makes most CLI languages statically typed (with or without type inference). However, starting with .NET Framework 4.0, the Dynamic 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, and implementations are available for other operating systems (see Silverlight and § Alternative implementations). Microsoft submitted the specifications for CLI (which includes the core class libraries, CTS, and CIL), C#, and C++/CLI to both Ecma International (ECMA) and International 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.
.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. Other code can demand that calling code be granted a specified permission. The demand causes CLR to perform a call stack walk: every assembly of each method in the call stack is checked for the required permission; if any assembly is not granted the permission a security exception is thrown.
Managed CIL bytecode is easier to reverse-engineer than native code, unless obfuscated. .NET decompiler 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 of reflection. This creates concerns in the business community over the possible loss of trade secrets and the bypassing of license control mechanisms. To mitigate this, Microsoft has included Dotfuscator Community Edition with Visual Studio .NET since 2002. Third-party obfuscation tools are also available from vendors such as VMware, V.i. Labs, Xenocode, and Red Gate Software. Method-level encryption tools for .NET code are available from vendors such as SafeNet.
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 a graph 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 a garbage collector (GC) which runs periodically, on a separate thread 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 are non-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 and objects defined as local variables or method parameters currently in scope, and objects referred to by CPU registers. When GC runs, it pauses the application and then, for each object referred to in the root, it recursively enumerates all the objects reachable from the root objects and marks them as reachable. It uses CLI metadata and reflection 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. This is the mark phase. 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 then compacted together to make free space on the managed heap contiguous again. Any reference to an object invalidated by moving the object is updated by GC to reflect the new location. 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.
GC used by .NET Framework is also generational. Objects are assigned a generation. Newly created objects are tagged Generation 0. Objects that survive a garbage collection are tagged Generation 1. Generation 1 objects that survive another collection are Generation 2. The framework uses up to Generation 2 objects. 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. Thus, by eliminating older (and thus more likely to survive a collection) objects from the scope of a collection run, fewer objects need checking and compacting.
When an application is first launched, the .NET Framework compiles the CIL code into executable code using its just-in-time compiler, and caches the executable program into the .NET Native Image Cache. 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 the Native Image Generator utility to manually ahead-of-time compile and cache any .NET application.
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."
.NET Framework provides support for calling Streaming SIMD Extensions (SSE) via managed code from April 2014 in Visual Studio 2013 Update 2. However, Mono has provided support for SIMD Extensions as of version 2.2 within the Mono.Simd namespace in 2009. Mono's lead developer Miguel de Icaza has expressed hope that this SIMD support will be adopted by CLR's ECMA standard. Streaming SIMD Extensions have been available in x86 CPUs since the introduction of the Pentium III. Some other architectures such as ARM and MIPS also have SIMD extensions. In case the CPU lacks support for those extensions, the instructions are simulated in software.
.NET Framework is the predominant implementation of .NET technologies. 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 by patent 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." 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 in C# (though some developers were able to use VB.NET, albeit with an amount of hacking, and with limited functionalities) and debugging (in an emulator or on hardware), both using Microsoft Visual Studio. It also features a subset of .NET Framework Class Library (about 70 classes with about 420 methods), a GUI framework loosely based on WPF, and additional libraries specific to embedded applications.
.NET Core is an alternative Microsoft implementation of the managed code framework; it has similarities with .NET Framework and even shares some API, but is designed based on different sets of principles: It is cross-platform and free and open-source.
Mono is an implementation of CLI and FCL, and provides added functions. It is dual-licensed as free and proprietary software. 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 of DotGNU) 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.
Microsoft Shared Source Common Language Infrastructure is a non-free implementation of CLR. However, the last version runs on Microsoft 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 is an implementation of CLI and parts of FCL. It is free software using an open source MIT License.
.NET Core is a cross-platform free and open-source managed software framework similar to .NET Framework. It consists of CoreCLR, a complete cross-platform runtime implementation of CLR, the virtual machine that manages the execution of .NET programs. CoreCLR comes with an improved just-in-time compiler, called RyuJIT. .NET Core also includes CoreFX, which is a partial fork of FCL. While .NET Core shares a subset of .NET Framework APIs, it comes with its own API that is not part of .NET Framework. Further, .NET Core contains CoreRT, the .NET Native runtime optimized to be integrated into AOT compiled native binaries. A variant of the .NET Core library is used for UWP. .NET Core's command-line interface offers an execution entry point for operating systems and provides developer services like compilation and package management.
.NET Core supports four cross-platform scenarios: ASP.NET Core web apps, command-line apps, libraries, and Universal Windows Platform apps. It does not implement Windows Forms or WPF which render the standard GUI for desktop software on Windows. .NET Core is also modular, meaning that instead of assemblies, developers work with NuGet packages. Unlike .NET Framework, which is serviced using Windows Update, .NET Core relies on its package manager to receive updates.
.NET Core 1.0 was released on 27 June 2016, along with Visual Studio 2015 Update 3, which enables .NET Core development.
.NET Standard is a set of APIs to unify the .NET Framework, .NET Core, and Xamarin platforms.
Microsoft managed code frameworks and their components are licensed as follows: