Undocumented MessageBoxTimeOut function

By: Eddie Shipman

Abstract: Describes usage of undocumented MessageBoxTimeout function from user32.dll

There are lots of neat little things that are in many of the DLLs that Microsoft has installed in Windows. Most of them are documented in the Win32 API. However, there are a lot of them that are undocumented. This article shows how to use one of the undocumented functions available in user32.dll, MessageBoxTimeOut.

This type of functionality for a MessageBox has been requested on the Delphi newsgroups many times and there have been several solutions written. After being introduced in XP, this functionality is now available to developers using this undocumented API.

Since this function is not documented, it is not found in Windows.pas, so it has to be defined. It is identical to the MessageBox API definition except it has two more parameters, wLanguageID and dmMilliseconds.

function MessageBoxTimeOut(hWnd: HWND; lpText: PChar; lpCaption: PChar;
                           uType: UINT; wLanguageId: WORD; dwMilliseconds: DWORD): Integer; stdcall;
function MessageBoxTimeOutA(hWnd: HWND; lpText: PChar; lpCaption: PChar;                             uType: UINT; wLanguageId: WORD; dwMilliseconds: DWORD): Integer; stdcall;
function MessageBoxTimeOutW(hWnd: HWND; lpText: PWideChar; lpCaption: PWideChar;                             uType: UINT; wLanguageId: WORD; dwMilliseconds: DWORD): Integer; stdcall;
implementation
// this const is not defined in Windows.pas const   MB_TIMEDOUT = 32000;
function MessageBoxTimeOut; externaluser32 name 'MessageBoxTimeoutA'; function MessageBoxTimeOutA; external user32 name 'MessageBoxTimeoutA'; function MessageBoxTimeOutW; external user32 name 'MessageBoxTimeoutW';

Now, to call the function, it is as easy as setting the flags and making the call. There may be other results returned that I am not aware of besides the standard IDxxx return values and the MB_TIMEDOUT result defined above.

var
  iResult: Integer;
  iFlags: Integer;
begin
  // Define a MessagBox  with an OK button and a timeout of 2 seconds
  iFlags  := MB_OK or MB_SETFOREGROUND or MB_SYSTEMMODAL or MB_ICONINFORMATION;
  iResult := MessageBoxTimeout(Application.Handle, 'Test a timeout of 2 seconds.',
                             'MessageBoxTimeout Test', iFlags, 0, 2000);
  // iResult will = 1 (IDOK)
  ShowMessage(IntToStr(iRet));

  // Define a MessageBox with a Yes and No button and a timeout of 5 seconds
  iFlags  := MB_YESNO or MB_SETFOREGROUND or MB_SYSTEMMODAL or MB_ICONINFORMATION;
  iResult := MessageBoxTimeout(Application.Handle, 'Test a timeout of 5 seconds.',
                              'MessageBoxTimeout Test', iFlags, 0, 5000);
  // iResult = MB_TIMEDOUT if no buttons clicked, otherwise
  // iResult will return the value of the button clicked
  case iResult of
  IDYES:  // Pressed Yes button
    ShowMessage('Yes');
  IDNO:  // Pressed the No button
    ShowMessage('No');
  MB_TIMEDOUT: // MessageBox timed out
    ShowMessage('TimedOut');
  end;
end;

I presume Borland will not put this into Windows.pas until Microsoft documents it but developers can get a head start on them by using the code above. It is unlikely that Microsoft will depricate this function for quite some time because all of the standard MessageBox API calls actually call MessageBoxTimeOutA or MessageBoxTimeoutW and pass $FFFFFFFF as the timeout period meaning the dialog will wait a very long time, approx 49 days!


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