COM interfaces are Immutable
Interface is not allowed to change
The phrase "COM interfaces are immutable" means that once a COM interface is published or deployed, its specification is not allowed to change. This applies even if different COM objects implement that interface.
For example, once interface
IX1 is published or deployed, all COM objects that implement
must implement the interface exactly as specified. So if
IX1 has the following IDL (interface definition language) definition:
interface IX1 : IUnknown
HRESULT x1(int ix);
vtable aka virtual table:
For every class that contains virtual functions, the compiler constructs a virtual table, vtable. The vtable contains an entry for each virtual function accessible by the class and stores a pointer to its definition. Only the most specific function definition callable by the class is stored in the vtable.