DISPIDs

DISPIDs, or dispatch identifiers, are integer identifiers used in Component Object Model (COM) automation to identify members of an interface that supports IDispatch. They enable late binding by letting clients invoke methods or access properties without compile-time knowledge of the interface.

In practice, a scripting or automation client calls IDispatch::GetIDsOfNames to map a member name (such as a method or property name) to its corresponding_DISPID, and then uses IDispatch::Invoke with that DISPID to call the member or read/write a property. This mechanism supports dynamic discovery and invocation of members at runtime, which is essential for languages like VBScript, VBA, and JavaScript in Windows environments.

DISPIDs can be assigned in two ways. They may be automatically assigned by a type library when the interface is described in IDL or in the type library itself, or they can be explicitly specified in the interface definition using attributes such as [dispid(n)]. The values are stable for a given type library, so clients relying on those values will continue to work as long as the library is unchanged. Some DISPIDs are reserved for standard purposes, such as DISPID_VALUE (often 0) for the default member, DISPID_PROPERTYPUT (-3) for property assignment, and DISPID_NEWENUM (-4) for enumerators.

If an object does not expose IDispatch (for example, a purely vtable-bound interface), DISPIDs are not used. Changes to DISPIDs can affect compatibility with automation clients, so versioning and type-library management are important considerations in COM development.