Question

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How to cast a Interface to a Object in Delphi

Using Delphi 5; I have an interface that I cannot change for legacy reasons. I am passing (pointers to) that interface all over the place. The implementing class has several new properties – is there a way to force a cast from the interface to the actual implementation?

http://www.malcolmgroves.com/blog/?p=500 says that this is (newly) implemented in Delphi 2010, which strongly suggest that it wasn’t possible before. Is this indeed the case, or is there a way I’m not familiar with? RTTI, maybe?

(I checked, and if pScore is TOleScore then is indeed not allowed by the Delphi 5 compiler – here pScore is my pScore: IScore argument, and TOleScore is the implementing class.)

Answer 1

• The classic take on this is by Hallvard Vassbotn: Hack #7: Interface to Object
• More recently Barry Kelly, a Delphi compiler engineer, also provided an implementation: An ugly alternative to interface to object casting

I think both approaches should work.

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Incidentally, does anyone know if Hallvard is still active? I’ve not come across him in the past few years.