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What is .NET? Introduction and overview

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.NET is a free, cross-platform, open source developer platform for building many kinds of applications. .NET is built on a high-performance runtime that is used in production by many high-scale apps.

Cloud apps

Cross-platform client apps

Windows apps

Other app types

Features

.NET features allow developers to productively write reliable and performant code.

Using .NET

.NET apps and libraries are built from source code and a project file, using the .NET CLI or an Integrated Development Environment (IDE) like Visual Studio.

The following example is a minimal .NET app:

Project file:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net7.0</TargetFramework>
  </PropertyGroup>
</Project>

Source code:

Console.WriteLine("Hello, World!");

The app can be built and run with the .NET CLI:

% dotnet run
Hello, World!

It can also be built and run as two separate steps. The following example is for an app that is named app:

% dotnet build
% ./bin/Debug/net6.0/app 
Hello, World!

Binary distributions

  • .NET SDK -- Set of tools, libraries, and runtimes for development, building, and testing apps.
  • .NET Runtimes -- Set of runtimes and libraries, for running apps.

You can download .NET from:

Free and open source

.NET is free, open source, and is a .NET Foundation project. .NET is maintained by Microsoft and the community on GitHub in several repositories.

.NET source and binaries are licensed with the MIT license. Additional licenses apply on Windows for binary distributions.

Support

Microsoft supports .NET on Android, Apple, Linux, and Windows operating systems. It can be used on Arm64, x64, and x86 architectures. It's also supported in emulated environments, like macOS Rosetta 2.

New versions of .NET are released annually in November. .NET releases in odd-numbered years are Long-Term Support (LTS) releases and are supported for three years. Releases in even-numbered years are Standard-Term Support (STS) releases and are supported for 18 months. The quality level, breaking change policies, and all other aspects of the releases are the same. For more information, see Releases and support.

The .NET Team at Microsoft works collaboratively with other organizations to distribute and support .NET in various ways.

Red Hat supports .NET on Red Hat Enterprise Linux (RHEL).

Samsung supports .NET on Tizen platforms.

Runtime

The Common Language Runtime (CLR) is the foundation all .NET apps are built on. The fundamental features of the runtime are:

  • Garbage collection.
  • Memory safety and type safety.
  • High level support for programming languages.
  • Cross-platform design.

.NET is sometimes called a "managed code" runtime. It's called managed primarily because it uses a garbage collector for memory management and because it enforces type and memory safety. The CLR virtualizes (or abstracts) various operating system and hardware concepts, such as memory, threads, and exceptions.

The CLR was designed to be a cross-platform runtime from its inception. It has been ported to multiple operating systems and architectures. Cross-platform .NET code typically does not need to be recompiled to run in new environments. Instead, you just need to install a different runtime to run your app.

The runtime exposes various diagnostics services and APIs for debuggers, dumps and tracing tools, and observability. The observability implementation is primarily built around OpenTelemetry, enabling flexible application monitoring and site reliability engineering (SRE).

The runtime offers low-level C-style interop functionality, via a combination of P/Invoke, value types, and the ability to blit values across the native/managed-code boundary.

Languages

The runtime is designed to support multiple programming languages. C#, F#, and Visual Basic languages are supported by Microsoft and are designed in collaboration with the community.

  • C# is a modern, object-oriented, and type-safe programming language. It has its roots in the C family of languages and will be immediately familiar to C, C++, Java, and JavaScript programmers.

  • F# is an interoperable programming language for writing succinct, robust, and performant code. F# programming is data-oriented, where code involves transforming data with functions.

  • Visual Basic uses a more verbose syntax that is closer to ordinary human language. It can be an easier language to learn for people new to programming.

Compilation

.NET apps (as written in a high-level language like C#) are compiled to an Intermediate Language (IL). IL is a compact code format that can be supported on any operating system or architecture. Most .NET apps use APIs that are supported in multiple environments, requiring only the .NET runtime to run.

IL needs to be compiled to native code in order to execute on a CPU, for example Arm64 or x64. .NET supports both Ahead-Of-Time (AOT) and Just-In-Time (JIT) compilation models.

  • On Android, macOS, and Linux, JIT compilation is the default, and AOT is optional (for example, with ReadyToRun).
  • On iOS, AOT is mandatory (except when running in the simulator).
  • In WebAssembly (Wasm) environments, AOT is mandatory.

The advantage of the JIT is that it can compile an app (unmodified) to the CPU instructions and calling conventions in a given environment, per the underlying operating system and hardware. It can also compile code at higher or lower levels of quality to enable better startup and steady-state throughput performance.

The advantage of AOT is that it provides the best app startup and can (in some cases) result in smaller deployments. The primary downside is that binaries must be built for each separate deployment target (the same as any other native code). AOT code is not compatible with some reflection patterns.

Runtime libraries

.NET has a comprehensive standard set of class libraries. These libraries provide implementations for many general-purpose and workload-specific types and utility functionality.

Here are some examples of types defined in the .NET runtime libraries:

For more information, see the Runtime libraries overview.

NuGet Package Manager

NuGet is the package manager for .NET. It enables developers to share compiled binaries with each other. NuGet.org offers many popular packages from the community.

Tools

The .NET SDK is a set of libraries and tools for developing and running .NET applications. It includes the MSBuild build engine, the Roslyn (C# and Visual Basic) compiler, and the F# compiler. Most commands are run by using the dotnet command. The CLI tools can be used for local development and continuous integration.

The Visual Studio family of IDEs offer excellent support for .NET and the C#, F#, and Visual Basic languages.

GitHub Codespaces and GitHub security features support .NET.

Notebooks

.NET Interactive is a group of CLI tools and APIs that enable users to create interactive experiences across the web, markdown, and notebooks.

For more information, see the following resources:

CI/CD

MSBuild and the .NET CLI can be used with various continuous integration tools and environments, such as:

For more information, see Use the .NET SDK in Continuous Integration (CI) environments.

Deployment models

.NET apps can be published in two different modes:

  • Self-contained apps include the .NET runtime and dependent libraries. They can be single-file or multi-file. Users of the application can run it on a machine that doesn't have the .NET runtime installed. Self-contained apps always target a single operating system and architecture configuration.
  • Framework-dependent apps require a compatible version of the .NET runtime, typically installed globally. Framework-dependent apps can be published for a single operating system and architecture configuration or as "portable," targeting all supported configurations.

.NET apps are launched with a native executable, by default. The executable is both operating system and architecture-specific. Apps can also be launched with the dotnet command.

Apps can be deployed in containers. Microsoft provides container images for various target environments.

.NET history

In 2002, Microsoft released .NET Framework, a development platform for creating Windows apps. Today .NET Framework is at version 4.8 and remains fully supported by Microsoft.

In 2014, Microsoft introduced .NET Core as a cross-platform, open-source successor to .NET Framework. This new implementation of .NET kept the name .NET Core through version 3.1. The next version after .NET Core 3.1 was named .NET 5.

New .NET versions continue to be released annually, each a major version number higher. They include significant new features and often enable new scenarios.

.NET ecosystem

There are multiple variants of .NET, each supporting a different type of app. The reason for multiple variants is part historical, part technical.

.NET implementations (historical order):

  • .NET Framework -- It provides access to the broad capabilities of Windows and Windows Server. Also extensively used for Windows-based cloud computing. The original .NET.
  • Mono -- A cross-platform implementation of .NET Framework. The original community and open source .NET. Used for Android, iOS, and Wasm apps.
  • .NET (Core) -- A cross-platform and open source implementation of .NET, rethought for the cloud age while remaining significantly compatible with .NET Framework. Used for Linux, macOS, and Windows apps.

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