Question

The official documentation says they are optional. I know COM interop requires a unique identifier for each interface but every interface example I see has a GUID whether it’s used with COM or not? Is there any benefit to including a GUID if its not going to be used with COM?

Answer 1

I’ve noticed that some methods such as Supports (to determine if a class conforms to a specific interface) require that you define a GUID before you can use them.

This page confirms it with the following information:

Note: The SysUtils unit provides an
overloaded function called Supports
that returns true or false when class
types and instances support a
particular interface represented by a
GUID. The Supports function is used in
the manner of the Delphi is and as
operators. The significant difference
is that the Supports function can take
as the right operand either a GUID or
an interface type associated with a
GUID, whereas is and as take the name
of a type. For more information about
is and as, see Class References.

Here’s some interesting information about interfaces, which states:

Why does an interface need to be
uniquely identifiable? The answer is
simple: because Delphi classes can
implement multiple interfaces. When an
application is running, there has to
be a mechanism that will get pointer
to an appropriate interface from an
implementation. The only way to find
out if an object implements an
interface and to get a pointer to
implementation of that interface is
through GUIDs.

Emphasis added in both quotes.

Reading this entire article also makes you realize that QueryInterface (which requires a GUID) is used behind the scenes for reasons such as reference counting.