Delphi: determine actual type of a generic?

Question

Is there any way to determine the type of a variable passed as an argument to a method? Consider the class:

TSomeClass = class
  procedure AddToList<T: TDataType; U: TListClass<T>>(Element: T; List: U);
end;

with the method implementation

procedure TSomeClass.AddToList<T, U>(Element: T; List: U);
begin
  if Element is TInt then
    List.AddElement(TInt.Create(XXX))
  else if Element is TString then
    List.AddElement(TString.Create(YYY));
end;

where TInt.Create() and TString.Create() have different sets of arguments, yet, they both inherit from TDataType.

Now, I know the is-operator can't be used like this, but is there a legal alternative that does what I'm asking here?

Answer 1

Not being able to use the is operator here is a known issue, but there's a pretty simple workaround.

  if TObject(Element) is TInt then
    List.AddElement(TInt.Create(XXX))

Also, since the type of a generic is part of the class and is known at compile-time, you might be better off restructuring your code. Make two different generic classes, one of which accepts a TInt as its <T> parameter, and the other of which accepts a TString. Put the type-specific functionality into them at that level, and have them descend from a common ancestor for shared functionality.

Answer 2

This question I asked some time ago

http://stackoverflow.com/questions/805931/conditional-behaviour-based-on-concrete-type-for-generic-class

might be of interest, especially if you want to use not only TObject descendants but also primitive types in your conditionals.