This project is an attempt to provide API consistency between the different .NET framework versions and to avoid the #if-#else-#endif preprocessor hell.

This library refers the .NET reference source repository and 3rd party libraries to obtain implementations for features that are missing in earlier framework releases. See the Licensing section for more information.

The project is divided into libraries, each providing different aspects that are normally found in .NETStandrd-compliant version of the .NET framework. The below table provides more information on how the shimmed features are organized:

Since the introduction of .NET Standard, it has become clear that the .NET world was undergoing a great effort of consolidation for the vast range of APIs and pursuing better platform coverage. Because the job was complicated, it lead to the creation of a few .NET Standard / .NET Core versions with increasing set of supported APIs with each release, having .NET Standard 2.0 as the latest stable release (at the moment of creating this document). .NET Standard 2.0 is now being way closer to the .NET Framework's set of APIs than it's predecesors.

NETStandard.Shim aims to encourage following the standartization trend, by exposing a number of the .NET Standard 2.0 APIs to earlier versions of the .NET Standard, as well as to earlier versions of the .NET Framework itself. The main goal of this project is to provide working polyfills for a couple of API gaps between the current .NET Standard version, earlier .NET Standard versions and .NET Framework versions not conforming to the standard completely. It also brings few of the new .NET goodies to earlier .NET Framework versions.

The naming of different .NET tools and frameworks that will be used in this document can be confusing to some folks (including me at first). Here is some quick reference to avoid ambiguity and confusion:

• .NET Framework
  This is the original .NET framework, that is available as a Windows installation. It targets Windows exclusively and different versions of it are pre-installed in the Windows OS. To a great extent, the Mono project provides an analogue for Linux and Mac OS, and Xamarin -- for the mobile operating systems (Androud and iOS). Framework monikers use the format netXX, where XX is the version of the framework, for example net20, net451 and etc.

• .NET Standard
  This is the standardized version of the framework, with reduced (and at places slightly changed) APIs, which covers all the various platforms. This is not a version of the framework by itself, rather an abstract set of APIs that are guaranteed to work on all the platforms that can run .NET Core or .NET Framework (since certain versions). Framework monikers use the format netstandardX.X where X.X represents the particular standard version, for example netstandard2.0.
  .NETStandard is intended to be the primary choice for developing libraries, because it guarantees higher portability.

• .NET Core
  A cross-platform open-source variation of the framework. This is the result of an effort to consistently target many platforms, and thus is cut off from its windows-only roots that we see in .NET Framework. That is why targeting this framework may make certain apis typical to .NET Framework unavailable. .NET Core is available as a standalone package for the major OS-es. Framework monikers use the format netcoreapp.

• Unified .NET Platform
  This version represents the merging of the .NET Standard and the .NET Core concepts, and is also a cross-platform .NET version.
  The first release that represents this unifucation is .NET 5, which uses the moniker net5.0, or net5.0-XX where XX is the target plafrom (e.g. windows) for platform-specific builds. Future versions (e.g. net6.0) of .NET will abide to this naming format and will always be compatible with netstandard2.1.

This project is licensed under the MIT license for maximum compatbility with the .NET reference source codebase, which is also referenced as a submodule. Please, refer to the submodules/referencesource/LICENSE.txt and submodules/referencesource/PATENTS.TXT for information about the respective license and patents.

Over the years, popular 3rd party libraries have attempted to address the challenge for consistent .NET apis across different .NET versions. The work invested in those projects should be respected. We are doing our best to re-use those libraries in this project as "reinventing the whell" is rarely a productive way of work. However, most of the existing polyfill projects were written by different people addressing different concerns in different times. This was an uncoordinated effort, and when these libraries are put together in a single project (this project), they don't always play nice with each other.

Below is a list of 3rd party libraries that have been chosen as depedencies of this project. The reasons for these choices were that they play nice (enough) with each other, and also, their licenses allow combining them together.

• LinqBridge, licensed under the BSD 3-Clause License
  This library has remained stable over the years and a useful extension for compatibility with net35

• NetLegacySupport.Tuple, licensed under the MIT License

• Portable.ConcurrentDictionary, licensed under MIT License

• System.Threading.Tasks.Unofficial. Various licenses available:

  • Apache-2.0 License
  • GPL-MIT License
  • LGPL-2.0 License
  • MIT License
  • MS-PL License

  We favored this library over the official TPL library from Microsoft, because its API surface was richer and closer to a complete polyfill.