根据值比较两个COM接口实例

Question

是否存在关于如何根据两个COM对象的值（即私有状态）而不是其标识（即指向IUnknown的指针）比较两个COM对象的模式？

Answer 1

Object equality is a heavy implementation detail, a detail that's well hidden in COM. COM is an interface based object model, the fact that an implementation is required of those interfaces is carefully hidden. It comes up in just a few places, CoCreateInstance() being the obvious one. Less obvious are the rules for IUnknown. Whose proper implementation requires that you'll get the same IUnknown interface pointer when you QI through any of the implemented interfaces. This provides object identity, not object equality.

There is no standard COM interface type that is commonly used to test object equality. You'll just have to make your own. A simple one with an IsEqualTo() method gets the job done. Or you could just add that method to your default interface.

Answer 2

It depends on definition of "private state". A COM object does not have to have one. If the objects implement IPersist* family of interfaces, esp. IPersistStreamInit , then you can save both into persistent stream or property bag, and then compare storages directly, such as byte-by-byte comparison of the streams.