midl

Microsoft Interface Definition Language (MIDL) is a description language for defining interfaces, data types, and marshalling rules that enable communication between software components, especially across process or network boundaries. It is widely used with Windows technologies such as COM and DCE/RPC to generate language- and platform-neutral stubs and type libraries from a single specification.

Developers write MIDL in .idl files, and the MIDL compiler (midl.exe) processes them to produce C/C++ header and source files, as well as proxy/stub code and, for COM interfaces, type libraries (.tlb). The generated code enables marshalling of complex types, parameter direction (in, out, in/out), and versioned interfaces.

Syntax and features include definitions of interfaces and their methods, a range of data types (structures, enums, unions, arrays, pointers), and attributes to control memory layout and marshalling (such as size_is, length_is, in, out). Interfaces are often annotated with GUIDs (IIDs) for COM or RPC interfaces, and types may be declared to participate in type libraries.

Output artifacts include the generated header files used by the client and server, the proxy/stub code for RPC, and type libraries used by COM clients. The MIDL compiler is part of the Windows SDK and is primarily targeted at Windows development, though the underlying concepts mirror interface definition languages used in other RPC ecosystems.

See also: Interface Definition Language, DCE/RPC, COM, Type Library.