如何使用“is”来检查类或确切的类(不是后代)?

type

TBase = class(TObject)
end;

TDerived = class(Tbase)
end;

procedure TForm1.Button1Click(Sender: TObject);
var
 A: TBase;

begin
  A:= TBase.Create;
  if A.ClassType = TBase then ShowMessage('TBase');  // shown
  A.Free;
  A:= TDerived.Create;
  if A.ClassType = TBase then ShowMessage('TBase again'); // not shown
  A.Free;
end;

begin
  A:= TBase.Create;
  if A.ClassType = TBase then ShowMessage('TBase');  // shown
  if PPointer(A)^ = TBase then ShowMessage('TBase');  // shown
  A.Free;
  A:= TDerived.Create;
  if PPointer(A)^ = TBase then ShowMessage('TBase again'); // not shown
  if A.ClassType = TBase then ShowMessage('TBase again');  // not shown
  A.Free;
end;

class function TBase.IsDerivedClass: boolean;
begin
  result := self=TDerivedClass;
end;

更好的方法;

type

TBase = class(TObject)
end;

TDerived = class(Tbase)
end;

procedure TForm1.Button1Click(Sender: TObject);
var
 A, B: TObject;

begin
  A:= TBase.Create;
  if A.ClassType = TBase then ShowMessage('A is Exacly TBase');  // shown
  if A is TBase then ShowMessage('A is TBase or SubClass of TBase');  // shown
  if A is TDerived then ShowMessage('A is TDerived or SubClass of TDerived ');  // NOT shown!! 
  A.Free;

  B:= TDerived.Create;
  if B.ClassType = TBase then ShowMessage('TBase again'); // not shown
  if B is TBase then ShowMessage('B is TBase or SubClass of TBase');  // shown
  if B is TDerived then ShowMessage('B is TDerived or SubClass of TDerived ');  // shown!! 
  B.Free;
end;