That's right - but by passing a reference/pointer to an IUnknown instance you are limited to only using its interface unless you re-cast it back to the original subclass. The only interfaces that the IUnknown interface provides are AddRef and Release, so I don't think this would be a very good way to go though.

This seems fundamentally unsafe to me. In order to do anything useful with the IUnknown you're talking about having to cast it back to it's original interface, but you don't know what the original interface was. Sure, you may say that "I only put vertex shaders into this member so this IUnknown must be a vertex shader", but that fails to protect you against programmer error, which really misses the point of stronger typing.

You don't have to cast it directly back to its original interface, you can use QueryInterface.

You don't have to cast it directly back to its original interface, you can use QueryInterface.

That would be safer, yes, but at the expense of having to Release the returned interface after every use.