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);
All COM objects that implement
IX1 must implement
vtable order with the specified parameters.
This does not mean that
IX1 will have the same implementation in different COM objects. It means only that the
vtable ordering and method parameters remain the same.
Consider the case of
IUnknown. All COM objects implement
vtable order with exactly the same parameters. However, a call to
QueryInterface in one object can have a different implementation than a call to
QueryInterface in another object because the objects probably support different interfaces.