Common Intermediate Language

In keeping with the Common Language Infrastructure name, another important feature of the CIL and the CLI is to support the interaction of multiple languages within the same application (instead of portability of source code across multiple operating systems). As a result, the CIL is the intermediate language not only for C# but also for many other languages, including Visual Basic .NET, the Java-like language J#, some incantations of Smalltalk, C++, and a host of others (more than 20 at the time of this writing, including versions of COBOL and FORTRAN). Languages that compile to the CIL are termed source languages, and each has a custom compiler that converts the source language to the CIL. Once compiled to the CIL, the source language is insignificant. This powerful feature enables the development of libraries by different development groups across multiple organizations, without concern for the language choice of a particular group. Thus, the CIL enables programming language interoperability as well as operating system portability.

A powerful feature of the CLI is its support for multiple languages. This support enables the creation of programs using multiple languages and the accessibility of libraries written in one language from code written in a different language.
Common Type System

Regardless of the programming language, the resultant program operates internally on data types; therefore, the CLI includes the Common Type System (CTS). The CTS defines how types are structured and laid out in memory, as well as the concepts and behaviors that surround types. It includes type manipulation directives alongside the information about the data stored within the type. The CTS standard applies to how types appear and behave at the external boundary of a language because the purpose of the CTS is to achieve interoperability between languages. It is the responsibility of the runtime at execution time to enforce the contracts established by the CTS.

Within the CTS, types are classified into two categories:

Values are bit patterns used to represent basic types, such as integers and characters, as well as more complex data in the form of structures. Each value type corresponds to a separate type designation not stored within the bits themselves. The separate type designation refers to the type definition that provides the meaning of each bit within the value and the operations that the value supports.
Objects contain within them the object’s type designation. (This helps in enabling type checking.) Objects have identity that makes each instance unique. Furthermore, objects have slots that can store other types (either values or object references). Unlike with values, changing the contents of a slot does not change the identity of the object.

These two categories of types translate directly to C# syntax that provides a means of declaring each type.

Common Language Specification

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), whose focus is on library implementations. The CLS is intended for library developers and provides 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, then, any libraries that are to be accessible from multiple languages should conform to the CLS. Note that the CLS is not concerned with types that are not exposed externally to the assembly.

Also note that it is possible to have the compiler issue a warning when you create an API that is not CLS-compliant. To accomplish this, you use the assembly attribute System.CLSCompliant and specify a value of true for the parameter.

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