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Custom interfaces are user-defined interfaces that allow clients to invoke interface methods based on their order in the VTable and knowledge of the argument types. The VTable lists the addresses of all the properties and methods that are members of the object, including the member functions of the interfaces that it supports. If the object does not support IDispatch, the entries for the members of the object's custom interfaces immediately follow the members of IUnknown.
If the object has a type library, you can access the custom interface through its VTable layout, which you can get using the Type Library Editor. If the object has a type library and also supports IDispatch, a client can also get the dispIDs of the IDispatch interface and bind directly to a VTable offset. Delphi's type library importer (TLIBIMP) retrieves dispIDs at import time, so clients that use dispinterfaces can avoid calls to GetIDsOfNames; this information is already in the _TLB unit. However, clients still need to call Invoke.