.NET class libraries

Class libraries are the shared library concept for .NET. They enable you to componentize useful functionality into modules that can be used by multiple applications. They can also be used as a means of loading functionality that is not needed or not known at application startup. Class libraries are described using the .NET Assembly file format.

There are three types of class libraries that you can use:

• Platform-specific class libraries have access to all the APIs in a given platform (for example, .NET Framework on Windows, Xamarin iOS), but can only be used by apps and libraries that target that platform.
• Portable class libraries have access to a subset of APIs, and can be used by apps and libraries that target multiple platforms.
• .NET Standard class libraries are a merger of the platform-specific and portable library concept into a single model that provides the best of both.

Platform-specific class libraries

Platform-specific libraries are bound to a single .NET platform (for example, .NET Framework on Windows) and can therefore take significant dependencies on a known execution environment. Such an environment exposes a known set of APIs (.NET and OS APIs) and maintains and exposes expected state (for example, Windows registry).

Developers who create platform-specific libraries can fully exploit the underlying platform. The libraries will only ever run on that given platform, making platform checks or other forms of conditional code unnecessary (modulo single sourcing code for multiple platforms).

Platform-specific libraries have been the primary class library type for the .NET Framework. Even as other .NET implementations emerged, platform-specific libraries remained the dominant library type.

Portable class libraries

Portable libraries are supported on multiple .NET implementations. They can still take dependencies on a known execution environment, however, the environment is a synthetic one that's generated by the intersection of a set of concrete .NET implementations. Exposed APIs and platform assumptions are a subset of what would be available to a platform-specific library.

You choose a platform configuration when you create a portable library. The platform configuration is the set of platforms that you need to support (for example, .NET Framework 4.5+, Windows Phone 8.0+). The more platforms you opt to support, the fewer APIs and fewer platform assumptions you can make, the lowest common denominator. This characteristic can be confusing at first, since people often think "more is better" but find that more supported platforms results in fewer available APIs.

Many library developers have switched from producing multiple platform-specific libraries from one source (using conditional compilation directives) to portable libraries. There are several approaches for accessing platform-specific functionality within portable libraries, with bait-and-switch being the most widely accepted technique at this point.

.NET Standard class libraries

.NET Standard libraries are a replacement of the platform-specific and portable libraries concepts. They are platform-specific in the sense that they expose all functionality from the underlying platform (no synthetic platforms or platform intersections). They are portable in the sense that they work on all supporting platforms.

.NET Standard exposes a set of library contracts. .NET implementations must support each contract fully or not at all. Each implementation, therefore, supports a set of .NET Standard contracts. The corollary is that each .NET Standard class library is supported on the platforms that support its contract dependencies.

.NET Standard does not expose the entire functionality of .NET Framework (nor is that a goal), however, the libraries do expose many more APIs than Portable Class Libraries.

The following implementations support .NET Standard libraries:

• .NET Core
• .NET Framework
• Mono
• Universal Windows Platform (UWP)

For more information, see .NET Standard.

Mono class libraries

Class libraries are supported on Mono, including the three types of libraries described previously. Mono is often viewed as a cross-platform implementation of .NET Framework. In part, this is because platform-specific .NET Framework libraries can run on the Mono runtime without modification or recompilation. This characteristic was in place before the creation of portable class libraries, so was an obvious choice to enable binary portability between .NET Framework and Mono (although it only worked in one direction).