coclass

In the Component Object Model (COM) and its Interface Description Language (MIDL), a coclass is a type library construct that represents a concrete class that can be instantiated and that implements one or more interfaces. A coclass provides a mapping from a class identifier (CLSIDs) to the interfaces the object supports, and it can specify which interface is the default for activation.

A coclass is declared within a type library or IDL file and is associated with a unique GUID that becomes the CLSID used to create instances at runtime. It lists the interfaces that the object implements, including which interface is the default for clients that activate the object and which interfaces may be exposed to automation or scripting. Interfaces themselves have their own interface identifiers (IIDs), and a coclass ties these together to describe the concrete object's public surface.

Activation and usage follow a standard pattern: a client calls a function such as CoCreateInstance with the CLSID of the coclass to create an object. The COM runtime instantiates the object and returns a pointer to one of the supported interfaces, typically the coclass’s default interface. Clients may then use QueryInterface to obtain other interfaces implemented by the same object.

In type libraries, coclasses enable designers to describe the capabilities of COM components in a language- and language-neutral way. They also relate to scenarios involving proxies and stubs for remote or aggregated objects, and to variants that support both early-bound and late-bound (IDispatch) access. Overall, the coclass serves as the concrete embodiment of a COM class within the IDL and type library ecosystem.