Metadata is machine-readable information about a resource, or “data about data.” Such information might include details on content, format, size, or other characteristics of a data source. In .NET, metadata includes type definitions, version information, external assembly references, and other standardized information.

In order for two components, systems, or objects to interoperate with one another, at least one must know something about the other. In COM, this “something” is an interface specification, which is implemented by a component provider and used by its consumers. The interface specification contains method prototypes with full signatures, including the type definitions for all parameters and return types.

Only C/C++ developers could readily modify or use Interface Definition Language (IDL) type definitions—not VB or other developers, and more importantly, not tools or middleware. So Microsoft had to invent something other than IDL that everyone could use. This something was called a type library . In COM, type libraries allow a development environment or tool to read, reverse engineer, and create wrapper classes that are most appropriate and convenient for the target developer. Type libraries also allow runtime engines, such as the VB, COM, MTS, or COM+ runtime, to inspect types at runtime and provide the necessary plumbing or intermediary support for applications to use them. For example, type libraries support dynamic invocation and allow the COM runtime ...

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