How do I determine the type of the implementing object of an interface

I'm attempting to write a unit test for a simple factory class that creates one of several possible implementing objects and returns it as an interface reference.

DUnit has a built in procedure, CheckIs(AObject: TObject; AClass: TClass; msg: string), that based on its name and the parameters it accepts should fail the test if the object's class type doesn't match the expected one. The only problem is it requires an object reference not an interface reference.

So I'm trying to use CheckTrue and perform the comparison in the body of the test but I'm not as familiar with Delphi's type checking support as I am with C#'s.

I know the is operator is out of the question since it only works with object references.

CheckTrue(LMyInterfaceReference {comparison here} TMyClass);

Any suggestions?

BTW, I'm using开发者_StackOverflow Delphi 2009 so I don't have access to the new RTTI support added in 2010+.

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I'm wondering why you MUST have to test this... maybe you really don't have to.

But if knowing the underlying object of a Interface is a must, you have two choices:

• Add a method to the interface which returns the underlying object, just a TObject, and implement this in each class just by returning self.
• Hack a bit, for example using this Interface to object routine.

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If you don't like hacks and don't feel like upgrading to Delphi 2010+ you may use an interface like this:

IImplementingObjectInterface = interface
  function GetImplementingObject: TObject;
end;

Make sure your objects also implement this interface and use it to extract the implementing object. If you need to do this for a lot of objects you can define your own TInterfacedObject derivate that already implements this so you can simply change your inheritance and be done.

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Barry Kelly (one of the main Embarcadero Delphi Compiler Engineers) wrote a nice An ugly alternative to interface to object casting this week.

It answers your question.

The fun is that Hallvard Vassbotn wrote a very similar piece of code back in 2004.

From Delphi 2010 on, you can just use an is check or as cast to go back from interface references to object references.

--jeroen