How to extend a Generic Collection in Delphi?

Question

I have a Generic collection like this:

TFoo = class;

TFooCollection<T: TFoo> = class(TObjectDictionary<string, T>)
   procedure DoSomething;
end;

It works fine.

Now I need to extend TFooCollection like this:

TBar = class( TFoo );   

TBarCollection<T: TBar> = class(TFooCollection)
   procedure DoSomethingElse;
end;

And the compiler complains that TFooCollection isn't defined. As TBar is inheriting from TFoo, I would like to take advantage of TFooCollection methods (that would work with TFoo and TBar items) and do something else just with TBar Collections.

Is it possible in Delphi?

Answer 1

You knew how to extend the generic collection TObjectDictionary, so simply apply that same technique when extending the generic collection TFooCollection. TObjectDictionary doesn't specify a type by itself — you needed to provide values for its two generic type parameters. One you hard-coded to string, and the other you provided by forwarding the generic type parameter received in TFooCollection.

Likewise, when specifying the base type for TBarCollection, you can either provide a hard-coded value for the TFooCollection type parameter, or you can forward the parameter from TBarCollection. You probably want to do the latter:

type
  TBarCollection<T: TBar> = class(TFooCollection<T>)
  end;