Common Language Specification (CLS)

Since the language integration advantages provided by the CTS generally outweigh the costs of implementing it, the majority of source languages support the CTS. However, there is also a subset of CTS language conformance called the Common Language Specification (CLS). Its focus is toward library implementations. It targets library developers, providing them with standards for writing libraries that are accessible from the majority of source languages, regardless of whether the source languages using the library are CTS-compliant. It is called the Common Language Specification because it is intended to also encourage CLI languages to provide a means of creating interoperable libraries, or libraries that are accessible from other languages.

For example, although it is perfectly reasonable for a language to provide support for an unsigned integer, such a type is not included as part of the CLS. Therefore, developers implementing a class library should not externally expose unsigned integers because doing so would cause the library to be less accessible from CLS-compliant source languages that do not support unsigned integers. Ideally, therefore, any development of libraries that is to be accessible from multiple languages should conform to the CLS specification. Note that the CLS is not concerned with types that are not exposed externally to the assembly.