Hook Function

type
  TSaveRedir = packed record
    Addr: Pointer;
    Bytes: array[0..4] of Byte;
  end;
  PSaveRedir = ^TSaveRedir;
  
//

procedure RedirectCall(FromAddr, ToAddr: Pointer; SaveRedir: PSaveRedir);
var
  OldProtect: Cardinal;
  NewCode: packed record
    JMP: Byte;
    Distance: Integer;
  end;
begin
  if not VirtualProtect(FromAddr, 5, PAGE_EXECUTE_READWRITE, OldProtect) then
    RaiseLastWin32Error;
  if Assigned(SaveRedir) then
  begin
    SaveRedir^.Addr := FromAddr;
    Move(FromAddr^, SaveRedir^.Bytes, 5);
  end;
  NewCode.JMP := $E9;
  NewCode.Distance := PChar(ToAddr) - PChar(FromAddr) - 5;
  Move(NewCode, FromAddr^, 5);
  if not VirtualProtect(FromAddr, 5, OldProtect, OldProtect) then
    RaiseLastWin32Error;
end;

procedure UndoRedirectCall(const SaveRedir: TSaveRedir);
var
  OldProtect: Cardinal;
begin
  if not VirtualProtect(SaveRedir.Addr, 5, PAGE_EXECUTE_READWRITE, OldProtect) then
    RaiseLastWin32Error;
  Move(SaveRedir.Bytes, SaveRedir.Addr^, 5);
  if not VirtualProtect(SaveRedir.Addr, 5, OldProtect, OldProtect) then
    RaiseLastWin32Error;
end;

function MyNewMessageBox(Self: TApplication; const Text, Caption: PChar;
  Flags: Longint): Integer;
begin
  ShowMessage('New Messagebox');
  //....
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Application.MessageBox('Youll never see this Text ', '...', MB_OK);
end;


var
  S: TSaveRedir;

initialization
  RedirectCall(@TApplication.MessageBox, @MyNewMessageBox, @S);

finalization
  UndoRedirectCall(S);