What's the difference between casting using (Object as TClass) and TClass(Object)

Question

Got an issue where MyObj.classnameis(TMyClass.classname) is true and TMyClass(MyObj) works but (MyObj as TMyclass).doSomething throws a conversion error.

I don't really want any help with that junk, although if you want to put it in the comments that'd be super. I just would like to know what the difference between Obj As Class and Class(Obj) is.

Answer 1

An as-cast checks the actual object type to make sure the cast is valid, and raises an exception if it's not. A "hard cast" (TMyClass(MyObj) style) does not check, it just tells the compiler to assume the cast is valid.

If you've got a situation where ClassNameIs returns true but the as-cast fails, that means you have two different classes in two different units with the same name, and the as-cast is trying to cast to the wrong one. This also means that your hard-cast is casting to the wrong one, which could potentially lead to memory corruption.

Run a full project search for "TMyclass =" to see where your multiple declarations are, and either rename one of the classes or use a full definition (obj as MyUnit.TMyClass) so the compiler will know which class you're trying to cast to.

Answer 2

Addition to Mason's post: Instead of a searching through your code, you could also call a method like this one at the location that gives you problem.

function GetClassInheritance(Obj : TObject) : string;
var ClassRef : TClass;
begin
  Result := '';
  ClassRef := obj.ClassType;
  Result := ClassRef.ClassName;
  ClassRef := ClassRef.ClassParent;
  while assigned(ClassRef) do
  begin
    Result    := ClassRef.ClassName + '.' + Result;
    ClassRef  := ClassRef.ClassParent;
  end;

  Result := '(' + obj.ClassType.UnitName + ')' + Result
end;

That will return you a string formated as (UnitName)TObject.TPersistent.TComponent.... I don't remember seeing "ClassType.UnitName" in older version of delphi, so that part might not work with them, but the rest should.