.NET Framework

The .NET Framework (pronounced as "dot net") is a proprietary software framework developed by Microsoft that runs primarily on Microsoft Windows. It was the predominant implementation of the Common Language Infrastructure (CLI) until being superseded by the cross-platform .NET project. It includes a large class library called Framework Class Library (FCL) and provides language interoperability (each language can use code written in other languages) across several programming languages. Programs written for .NET Framework execute in a software environment (in contrast to a hardware environment) named the Common Language Runtime (CLR). The CLR is an application virtual machine that provides services such as security, memory management, and exception handling. As such, computer code written using .NET Framework is called "managed code". FCL and CLR together constitute the .NET Framework.

FCL provides the user interface, data access, database connectivity, cryptography, web application development, numeric algorithms, and network communications. Programmers produce software by combining their source 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 an integrated development environment for .NET software called Visual Studio.

.NET Framework began as proprietary software, although the firm worked to standardize the software stack almost immediately, even before its first release. Despite the standardization efforts, developers, mainly those in the free and open-source software communities, expressed their unease with the selected terms and the prospects of any free and open-source implementation, especially regarding software 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.

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 reliability bug 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 July 2025.

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

In August 2000, Microsoft and Intel worked to standardize Common Language Infrastructure (CLI) and C#. By December 2001, both were ratified by ECMA standards. 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.

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 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.

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