DispId

DISPID stands for Dispatch Identifier, a 32-bit integer used by the IDispatch interface in Microsoft’s Component Object Model (COM) to identify members of an Automation-compatible interface during late binding. Each named method or property exposed by a COM object that supports IDispatch is associated with a DISPID. Clients call IDispatch::Invoke with the DISPID to invoke methods or access properties. The mapping from names to DISPIDs is performed by IDispatch::GetIDsOfNames, which accepts a list of names and returns the corresponding DISPIDs. Once resolved, subsequent calls can use the numeric IDs directly.

Some DISPIDs have reserved meanings within the Automation model. For example, DISPID_VALUE (0) designates the default member, and DISPID_NEWENUM (-4) identifies the enumerator used to iterate a collection. DISPID_PROPERTYPUT (-3) is used when setting a property. Other standard or library-defined DISPIDs may exist, and many interfaces rely on explicit, library-defined IDs rather than the default ones.

DispIDs are typically defined in a type library or IDL. Developers can assign explicit values using the [dispId(n)] attribute (or [id(n)]) and can mark members as properties or methods with the appropriate attributes. The exact values are part of the interface contract; changing them breaks compatibility between components and consumers that rely on late binding. For dual interfaces, early binding uses v-table slots, while IDispatch-based clients use DISPIDs at runtime.