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Over the years, we've accumulated quite a few terms, platforms, and components that can make it hard for folks (including us) to understand what we're referring to. This document has a list that will help to qualify what we mean by what.

This will also list some aliases. As you'll see the aliases aren't always correct -- they are merely listed to help you find the better and less confusing terminology.


In this document, the following terms are used:

  • IL. Intermediate language. Higher level .NET languages, such as C#, compile down to a hardware agnostic instruction set, which is called Intermediate Language (IL). IL is sometimes referred to as MSIL (Microsoft IL) or CIL (Common IL).

  • GC. Garbage collector. Garbage collection is an implementation of automatic memory management. .NET Framework and .NET Core currently uses a generational garbage collector, i.e. it groups objects into generations to limit the number of nodes it has to walk for determining which objects are alive. This speeds up collection times.

  • JIT. Just in time compiler. This technology compiles IL to machine code that the processor understands. It's called JIT because compilation happens on demand and is performed on the same machine the code needs to run on. Since JIT compilation occurs during execution of the application, compile time is part of the run time. Thus, JIT compilers have to trade spending more time optimizing code with the savings the resulting code can produce. But a JIT knows the actual hardware and can free developers from having to ship different implementations. For instance, our vector library relies on the JIT to use the highest available SIMD instruction set.

  • AOT. Ahead of time compiler. Similar to JIT, this compiler also translates IL to machine code. In contrast to JIT compilation, AOT compilation happens before the application is executed and is usually performed on a different machine. AOT tool chains don't trade runtime for compile time and thus can spend more time optimizing. Since the context of AOT is the entire application, the AOT compiler can also perform cross module linking and whole program analysis, which means that all references are followed and a single executable is produced.

  • NGEN. Native (image) generation. You can think of this technology as a persistent JIT compiler. It usually compiles code on the machine where the code will be executed, but compilation typically occurs at install time.

  • CLR. Common language runtime. The exact meaning depends on context, but it usually refers to the runtime of the .NET Framework and includes several components. The CLR is a virtual machine, i.e. it includes the facilities to generate and compile code on-the-fly using a JIT compiler. The existing Microsoft CLR implementation is Windows only.

  • CoreCLR. Core common language runtime. It's built from the same code base as the CLR. Originally, CoreCLR was the runtime of Silverlight and was designed to run on multiple platforms, specifically Windows and OS X. CoreCLR is now part of .NET Core and represents a simplified version of the CLR. It's still a cross platform runtime. CoreCLR is also a virtual machine with a JIT.

  • CoreRT. Core runtime. In contrast to the CLR/CoreCLR, CoreRT is not a virtual machine, i.e. it doesn't include the facilities to generate and run code on-the-fly because it doesn't include a JIT. It does, however, include the GC and the ability for runtime type identification (RTTI) as well as reflection. However, its type system is designed so that metadata for reflection can be omitted. This enables having an AOT tool chain that can link away superfluous metadata and (more importantly) identify code that the application doesn't use.

  • CoreFX. Core framework. Conceptually a set of System.* (and to a limited extent Microsoft.*) libraries that make up the lower layer of the .NET library stack. It's what most people would think of as the Base Class Library (BCL). The BCL is a general purpose, lower level set of functionality that higher-level frameworks, such as WCF and ASP.NET, build on. The source code of the .NET Core library stack is contained in the CoreFX repo. However, the majority of the .NET Core APIs are also available in the .NET Framework, so you can think of CoreFX as a fork of the .NET Framework library stack.


.NET Framework

Also referred to as: Desktop, full framework, in-box framework

This refers to the .NET Framework that first shipped in 2002 and has been updated on a regular basis since then. It's the main framework folks target today and allows you to build a wide variety of applications, such as WinForms, WPF, ASP.NET, and command line tools.

The .NET Framework was designed to run on Windows only. Some versions of the .NET Framework come pre-installed with Windows, some require to be installed. However, in both cases the .NET Framework is a system-wide component. Applications do not include .NET Framework DLLs when deploying; the correct .NET version must be on the machine.

.NET Core

Also referred to as: UWP, Store

Originally, .NET Core was the identifier we used to describe the .NET APIs Windows 8 store applications could use. When we designed the API set, we wanted to create a foundation for .NET where portability is a first class concern for the layering and componentization. For more details, read this blog post.

Today, .NET Core is no longer just for store applications. .NET Core is the name for the open source, cross-platform stack that ASP.NET Core and UWP applications are built on. The stack includes a set of framework libraries (CoreFX), a JIT based runtime (CoreCLR), an AOT based runtime (CoreRT), and a set of tooling (such as the dotnet CLI).

That's why referring to .NET Core as 'Store' is no longer correct. But you can think of today's .NET Core as an evolution of the original APIs available for store applications. Many of the original design goals are still relevant, especially around layering and portability.

Universal Windows Platform (UWP)

Also referred to as: Store, WinRT, Metro

The Universal Windows Platform (UWP) is the platform that is used for building modern, touch-enabled Windows applications as well as headless devices for Internet of Things (IoT). It's designed to unify the different types of devices that you may want to target, including PCs, tablets, phablets, phones, and even the Xbox.

UWP provides many services, such as a centralized app store, an execution environment (AppContainer), and a set of Windows APIs to use instead of Win32 (WinRT). UWP has no dependency on .NET; apps can be written in C++, C#, VB.NET, and JavaScript. When using C# and VB.NET the .NET APIs are provided by .NET Core.

.NET Native

Also referred to as: ahead-of-time (AOT), IL compiler (ILC)

.NET Native is a compiler tool chain that will produce native code ahead-of-time (AOT), as opposed to just-in-time (JIT). The compilation can happen on the developer machine as well as on the store side, which allows blending AOT with the benefits of servicing.

You can think of .NET Native as an evolution of NGEN (Native Image Generator): NGEN basically simply runs the JIT up front, the code quality and behavior is identical to the JITed version. Another downside of NGEN is that it happens on the user's machine, rather than the developer's machine. NGEN is also at the module level, i.e. for each MSIL assembly there is a corresponding NGEN'ed assembly that contains the native code. .NET Native on the other hand is a C++ like compiler and linker. It will remove unused code, spend more time optimizing it, and produce a single, merged module that represents the closure of the application.

UWP was the first application model that was supported by .NET Native. We now also support building native console applications for Windows, OS X and Linux.


Also referred to as: Shared Source Common Language Infrastructure (SSCLI)

Pretty much at the same time the .NET Framework was released, Microsoft also published Rotor, which is the source code for an implementation of ECMA 335 (Common Language Infrastructure), which is the specification behind .NET.

While parts of the source were identical with the .NET Framework, many pieces had prototypic implementations instead: the purpose of Rotor wasn't to provide a production ready .NET implementation but to provide a platform for research, academia, and validation that the ECMA 335 specification itself can be implemented.

It's also worth pointing out that the source code of Rotor was not released under an open source license (i.e. not approved by OSI) and has not been officially updated since .NET Framework 2.0.


Mono is an open source alternative to the .NET Framework. Mono started around the same time the .NET Framework was first released. Since Microsoft didn't release Rotor as open source, Mono was forced to start from scratch and is thus a complete re-implementation of the .NET Framework with no shared code.

When .NET Core was released under the MIT license, Microsoft also released large chunks of the .NET Framework under the MIT license as well, which can be found here. This enabled the Mono community to use the same code the .NET Framework uses in order to close gaps and avoid behavioral differences.

Mono is primarily used to run .NET applications on Linux and Mac OS X (though to get into the Mac App Store you need Xamarin, see below). There are ports of Mono to other platforms, see Mono's Supported Platforms

Mono has implementations (though not necessarily complete) of WinForms, ASP.NET, and System.Drawing.


Xamarin is a commercial offering for building mobile applications targeting Android, iOS and Mac OS X Store. It's based on Mono, and on iOS and Android surfaces a different API profile, called the mobile profile. The subsetting was necessary to reduce the footprint, both by shipping smaller versions of the system libraries as well as making them more linker friendly. While Mono runs on Mac OS X without Xamarin, their linker is required make the app package for the Mac App Store. Xamarin ships a full static compiler on iOS, as the platform does not support dynamic code generation.


Language-Integrated Query

Also referred to as: LINQ

Introduced in .NET Framework 3.5, Language-Integrated Query's (LINQ) goal to make data processing easier. LINQ is primarily a collection of methods that extend IEnumerable and IEnumerable<T>. LINQ is intended to be used with extension methods and Lambda functions (added in C# 3.0 and VB 9.0 at the same time as .NET Framework 3.5 was released) allowing for a function style of programing.

A simple example of LINQ is

var odds = source.Where(obj => obj.Id == 1).ToArray();

IQueryable<T> and Expressions

One of the big advantages of using LINQ over more common data processing patterns is that the function given to the LINQ function can be converted to an expression and then executed in some other form, like SQL or on another machine across the network. An expression is a in-memory representation of some logic to follow.

For example, in the above sample source could actually be a database connection and the function call Where(obj => obj.Id == 1) would be conveted to a SQL WHERE clause: WHERE ID = 1, and then executed on the SQL server.

Parallel LINQ

Also referred to as: PLINQ

Also introduced in .NET Framework 3.5 Parallel LINQ. Parallel LINQ has a subset of the methods the LINQ does but may execute the iterations on different threads in any order. Generally to use Parallel LINQ you would just call the AsParallel() method on a collection implementing IEnumerable. And if at any point you wanted to return to "normal LINQ you can just call AsSequential().


Released in June 2006 by James Newton-King, JSON.NET has become the defacto standard for JSON serialization and deserialization in .NET. It is open source and support almost every platform .NET code can run on (.NET Framework 2.0, 3.0, 3.5, 4.0, and 4.5; Mono; MonoTouch/Xamarin.iOS; MonoDroid/Xamarin.Android; Silverlight 3, 4, and 5; Windows Phone 8, 8, and 8.1; Windows 8 Store; .NET Core).

Windows Forms

Also referred to as: WinForms

Windows Forms is an API provided by the .NET Framework (mostly in the System.Windows.Forms namespace) for creating desktop applications. Windows Forms provides an event-driven model for application development on top of the native loop-driven Win32 model. Mono has an implementation of Windows Forms, though it is not complete, since some parts of Windows Forms are tied to the Windows platform.

Windows Forms is in maintenance mode now. That means new features will generally not be added.

Windows Presentation Foundation

Also referred to as: WPF, Avalon

Introduced in .NET Framework 3.0, Windows Presentation Foundation (WPF) was a new API for creating desktop applications. Like Windows Forms, WPF is event-driven. However, instead of using GDI/GDI+ for drawing applications, WPF used DirectX. Using DirectX allowed WPF applications to use the GPU for rendering, freeing the CPU for other tasks. WPF also introduced XAML, an XML-based language which allows a declarative way to describe user interfaces and data binding to models (XAML is used by Silverlight, UWP, and Xamarin as well).

The .NET platform currently doesn't contain a cross-platform XAML-based UI stack. There are, however, various community projects in that space:

  • CSHTML5: A product to compile WPF/.NET Framework applications into HTML5/CSS3/ECMAScript5 applications.
  • WPFLight: An OSS project to create WPF on top of XNA/MonoGame.
  • Perspex: A cross-platform UI framework based on WPF.
  • Granular: A OSS project to allow WPF applications to run in the browser.