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Common Language Infrastructure metadata about classes and class members

Metadata, in the Common Language Infrastructure (CLI), refers to certain data structures embedded within the Common Intermediate Language (CIL) code that describes the high level structure of the code. Metadata describes all classes and class members that are defined in the assembly, and the classes and class members that the current assembly will call from another assembly. The metadata for a method contains the complete description of the method, including the class (and the assembly that contains the class), the return type and all of the method parameters. A CLI language compiler will generate the metadata and store this in the assembly containing the CIL. When the run time executes CIL it will check to make sure that the metadata of the called method is the same as the metadata that is stored in the calling method. This ensures that a method can only be called with exactly the right number of parameters and exactly the right parameter types. The Windows Runtime application platform, present in Windows 8 and Windows Phone 8, makes use of the CLI metadata format to describe component interfaces for code written in any of the supported programming languages. A difference in use within the Common Language Runtime is that an assembly typically does not contain any CIL instructions.

Assemblies contain tables of metadata. These tables are described by the CIL specification. The metadata tables will have zero or more entries and the position of an entry determines its index. When CIL code uses metadata it does so through a metadata token. This is a 32 bit value where the top 8 bits identify the appropriate metadata table, and the remaining 24 bits give the index of the metadata in the table. The Framework SDK contains a sample called that will list the metadata tables in an assembly, however, this information is rarely of use to a developer. Metadata in an assembly may be viewed using the ILDASM tool provided by the NET Framework SDK. In the CIL standard, metadata is defined in ILAsm (assembly language) form, an on disk representation form for storage, and a form that is embedded into assemblies of the Portable Executable (PE, exe or dll) format. The PE form is based on the on disk form.

Reflection is the API used to read CLI metadata. The reflection API provides a logical view of metadata rather than the literal view provided by tools like metainfo. Reflection in version 1.1 of the NET framework can be used to inspect the descriptions of classes and their members, and invoke methods. However, it does not allow runtime access to the CIL for a method. Version 2.0 of the framework allows the CIL for a method to be obtained.

Besides the namespace, other tools are also available that can be used to handle metadata. The Microsoft NET Framework ships a CLR metadata manipulation library that is implemented in native code. Third party tools to retrieve and manipulate metadata include PostSharp and Mono Cecil can also be used.