Undeclared Identifier

i've got an elementary question, so elementary that i'm almost afraid to ask,..but i gotta do what i gotta do...i'm trying to customize the CreateMessageDialog function from the Dialogs unit. I've copied and pasted the function into my code and renamed it CreateMyMessageDialog. I have included the Dialogs unit in my uses clause. When i try to compile my code i get this error:

[Error] SwipeForm.pas(xxx): Undeclared identifier: 'TMessageForm'

on this line:

Result := TMessageForm.CreateNew(Application);

which is pretty much the first line of code in the function (snippet included), so what am i doing wrong

function TfrmSwipeEntry.CreateMyMessageDialog(const Msg: string; DlgType: TMsgDlgType;
  Buttons: TMsgDlgButtons): TForm;
const
  mcHorzMargin = 8;
  mcVertMargin = 8;
  mcHorzSpacing = 10;
  mcVertSpacing = 10;
  mcButtonWidth = 50;
  mcButtonHeight = 14;
  mcButtonSpacing = 4;
var
  DialogUnits: TPoint;
  HorzMargin, VertMargin, HorzSpacing, VertSpacing, ButtonWidth,
  ButtonHeight, ButtonSpacing, ButtonCount, ButtonGroupWidth,
  IconTextWidth, IconTextHeight, X, ALeft: Integer;
  B, DefaultButton, CancelButton: TMsgDlgBtn;
  IconID: PChar;
  TextRect: TRect;
begin
  Result := TMessageForm.CreateNew(Application); <--error happens here
  .
  .
  .
end;

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