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.NET is open source and under the .NET Foundation. The .NET Foundation is an independent organization to foster open development and collaboration around the .NET ecosystem.

Because .NET is open source, you can join the thousands of developers and companies already contributing to the .NET platform.

.NET is made up of a number of key components. It has a standard library, called the .NET Standard Library, which is a large set of APIs which run everywhere. The .NET Standard is a formal specification of .NET APIs that are intended to be available on all .NET implementations. The motivation behind the .NET Standard is establishing greater uniformity in the .NET ecosystem. This standard library is implemented by three .NET runtimes:

  1. .NET Framework: The .NET Framework is a development platform for building apps for web, Windows, Windows Phone, Windows Server, and Microsoft Azure. It consists of the common language runtime (CLR) and the .NET Framework class library, which includes a broad range of functionality and support for many industry standards.
  2. .NET Core: .NET Core is a general purpose development platform maintained by Microsoft and the .NET community on GitHub. It is cross-platform, supporting Windows, macOS and Linux, and can be used in device, cloud, and embedded/IoT scenarios.
  3. Mono for Xamarin: Mono is a .NET implementation that is mainly used when a small runtime is required. It is the runtime that powers Xamarin applications on Android, Mac, iOS, tvOS and watchOS and is focused primarily on a small footprint. Mono also powers games built using the Unity engine. It supports all of the currently published .NET Standard versions.

The .NET languages also run on any .NET runtime. Additionally, there are tools on every platform which allow you to build projects. These tools are the same regardless of your choice of runtime.

Here's a graphical overview of each of the previously mentioned components of .NET and how they fit.

Figure 1: .NET Architectural Components

.NET Framework 4/4.5

The .NET Framework is an integral Windows component that supports building and running the next generation of applications and XML Web services. The .NET Framework has two main components: the common language runtime and the .NET Framework class library. The common language runtime manages memory, thread execution, code execution, code safety verification, compilation, and other system services. The .NET Framework class library is a collection of reusable types that tightly integrate with the common language runtime.

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