Metadata provides the following major benefits:

• Self-describing files – Common language runtime modules and assemblies are self-describing. A module’s metadata contains everything needed to interact with another module. Metadata automatically provides the functionality of IDL in COM, allowing you to use one file for both definition and implementation. Runtime modules and assemblies do not even require registration with the operating system. As a result, the descriptions used by the runtime always reflect the actual code in your compiled file, which increases application reliability.

• Language interoperability and easier component-based design. – Metadata provides all the information required about compiled code for you to inherit a class from a PE file written in a different language. You can create an instance of any class written in any managed language (any language that targets the common language runtime) without worrying about explicit marshaling or using custom interoperability code.

• Attributes – The .NET Framework allows you to declare specific kinds of metadata, called attributes, in your compiled file. Attributes can be found throughout the .NET Framework and are used to control in more detail how your program behaves at run time. Additionally, you can emit your own custom metadata into .NET Framework files through user-defined custom attributes. For more information, see Extending Metadata Using Attributes.

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 metainfo 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.

The CLS provides following advantages:

1. Access to Complete .NET Framework hierarchy.
2. High level of interoperability within languages.(e.g. – C# Class can be inherit from a Visual Basic class).
3. Component is accessible from any programming language that supports the CLS.
4. All CLS-compliant languages can produce verifiable code so the type safety of code possible.

What is .NET metadata?

In the past, a software component (.exe or .dll) written in one language could not easily use a software component written in another language. COM provided a step forward in solving this problem. The .NET Framework makes component interoperation even easier by allowing compilers to emit additional declarative information into all modules and assemblies. This information, called metadata, helps components to seamlessly interact.

Metadata is binary information describing your program that is stored either in a common language runtime portable executable (PE) file or in memory. When you compile your code into a PE file, metadata is inserted into one portion of the file, while your code is converted to Microsoft intermediate language (MSIL) and inserted into another portion of the file.

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. When the CLR 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.

Metadata is the key to a simpler programming model, eliminating the need for Interface Definition Language (IDL) files, header files, or any external method of component reference. Metadata allows .NET languages to describe themselves automatically in a language-neutral manner, unseen by both the developer and the user. Additionally, metadata is extensible through the use of attributes.

The Common Language Specification (CLS), which is a set of basic language features needed by many .Net applications to fully interact with other objects regardless of the language in which they were implemented. The CLS represents the guidelines defined by for the .NET Framework. These specifications are normally used by the compiler developers and are available for all languages, which target the .NET Framework.

The CLS helps enhance and ensure language interoperability by defining a set of features that developer can rely on to be available in a wide variety of languages.