The IDispatch interface was defined so that a single, standard interface could be used by all components wanting to expose their functionality to interested clients. This interface, and the marshaling code built for it, are called OLE automation. IDispatch and VARIANTs are joined at the hip. As you would expect, calling methods through IDispatch is slower than over the custom interface.

The proxy-stub code needed to marshal the IDispatch interface is contained in the oleaut32.dll file; this is important because it means that applications implementing the IDispatch interface don’t have to provide their own marshaling code.

IDispatchEx extends IDispatch. It was added so scripting languages could add and remove properties and methods at runtime.

By the way, IDispatchEx is a good example of COM working the way it’s supposed to. If QueryInterface(IID_IDispatchEx, ...) returns E_NOINTERFACE, the object does not support adding items.

Server side

To declare a dispatch interface in IDL, use dispinterface. Not a best practice.

[ uuid(10000001-0000-0000-0000-000000000001) ]
dispinterface ISum
    [id(1)] int x;
    [id(2)] int y;

    [id(3)] int Sum(int x, int y);

To declare a dual interface, use interface with a dualattribute - but notice the other changes. Considered a best practice because… you might as well. Most clients will use the custom interface.

[ object, uuid(10000001-0000-0000-0000-000000000001),
  dual ]
interface ISum : IDispatch
    [id(1)] HRESULT Sum(int x, int y, [out, retval] int* retval);

The system provides helper functions (Disp*), see references.

Client side

Early vs. Late Binding

The client has 2 styles of calling. If you use the interface as designed, it’s called “late binding”. That means you call IDispatch::GetIDsOfNames, followed by IDispatch::Invoke. The DISPIDs can be optionally cached.

Early binding simply means that it gets the DISPID from the type library. (If perf is really an issue, consider using the custom interface.)