Interface Definition Language
As was described previously in this specification, the COM infrastructure is completely divorced from the source-level tools used to create and use COM components. COM is completely a binary specification, and thus source-level specifications and standards have no role to play in the fundamental architecture of the system.
Specifically, and somewhat different than other environments, this includes any and all forms of interface definition language (IDL). Having an interoperable standard for an appropriate IDL (or any other source level tool for that matter) is still incredibly valuable and useful, it's just important to understand that this is a tool standard and not a fundamental system standard. Contrast this, for example, with the DCE RPC API specification, where, if only because the fundamental SendReceive API is not part of the public standard runtime infrastructure, one must use IDL to interoperate with the system.1. People can (and have, out of necessity) built COM components with custom COM interfaces without using any interface definition language at all. This clear separation of system standards from tools standards is an important point, for without it COM tools vendors cannot innovate without centralizing their innovations through some central standards body. Innovation is stifled, and the customers suffer a loss of valuable tools in the marketplace.
That all being said, as was just mentioned, source-level standards are still useful, and DCE IDL is one such standard. The following enhancements to DCE IDL enable it to specify COM interfaces in addition to DCE RPC interfaces.2.