One of the obvious themes of .NET is unification and interoperability between various programming languages. In order to achieve this; certain rules must be laid and all the languages must follow these rules. In other words we can not have languages running around creating their own extensions and their own fancy new data types. CLS is the collection of the rules and constraints that every language (that seeks to achieve .NET compatibility) must follow.
The Common Language Specification is a set of basic language features (constructs and constraints) that serves as a guide for library writers and compiler writers. It allows libraries to be fully usable from any language supporting the CLS, and for those languages to integrate with each other. The Common Language Specification is a subset of the common type system. The CLS is actually a set of restrictions on the CTS. The CLS defines not only the types allowed in external calls, but the rules for using them, depending on the goal of the user.
Informally CLS is a simply a contract between programming language designers and class library authors. The CLS is basically just a subset of the entire set of features supported by the CLR. The CLS includes things such as calling virtual methods, overloading methods and does not include things such as unsigned types. CLS is weighted very heavily in favor of the library designers.
The Common Language Specification describes a common level of language functionality. The CLS is a set of rules that a language compiler must adhere to in order to create .NET applications that run in the CLR. Anyone who wants to write a .NET·-compliant compiler needs simply to adhere to these rules and that's it. The relatively high minimum bar of the CLS enables the creation of a club of CLS-compliant languages. Each member of this club enjoys dual benefits: complete access to .NET framework functionality and rich interoperability with other compliant languages. For example a VB class can inherit from a C# class and override its virtual methods.
The common language specification (CLS) is a collection of rules and restrictions that allow interoperation between languages. Even though the CLI does not require compilers to follow CLS, code that follows the CLS rules is compatible with all other languages that follow the CLS rules.
Microsoft has defined three level of CLS compatibility/compliance. The goals and objectives of each compliance level have been set aside. The three compliance levels with their brief description are given below:
The component developed in this type of language can be used by any other language.
The language in this category can use classes produced in any other language. In simple words this means that the language can instantiate classes developed in other language. This is similar to how COM components can be i:=1stantiated by your ASP code.
Languages in this category can not just use the classes as in CONSUMER category; but can also extend classes using inheritance.
Languages that come with Microsoft Visual Studio namely Visual C++, Visual Basic and
C#; all satisfy the above three categories. Vendors can select any of the above categories as the targeted compliance level(s) for their languages.