type TControlClass = class of TControl;
function CreateControl(ControlClass: TControlClass;

    const ControlName: string; X, Y, W, H: Integer): TControl;
begin
  Result := ControlClass.Create(MainForm);
  with Result do
  begin
    Parent := MainForm;
    Name := ControlName;
    SetBounds(X, Y, W, H);
    Visible := True;
  end;
end;

CreateControl(TEdit, 'Edit1', 10, 10, 100, 20);