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Disable the numlock key using Delphi 7

Posted on 2008-10-27
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Last Modified: 2013-11-23
I want to be able to use Delphi 7 to disable the numlock key.  This needs to work in XP and preferably Vista also.

Actually what I specifically want to do is to make sure the numlock key can't be turned off if that makes more sense?  setting it explicitly to on first, before turning off the key would therefore work well.
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Question by:AuthorwareXtras
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Expert Comment

by:Johnjces
ID: 22818674
You could try, to "click" the numlock key;

keybd_event(VK_NUMLOCK, 0, 0, 0);

Actually the above will "hold down" the key.

To click try;

keybd_event(VK_NUMLOCK, 0, KEYEVENTF_KEYDOWN, 0);

To reverse it, try;

keybd_event(VK_NUMLOCK, 0, KEYEVENTF_KEYUP, 0);

John
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Expert Comment

by:leei
ID: 22819122
This should be everything you need:
1. Method to set NumLock state
2. Keyboard hook to allow watching for attempts to change the NumLock state and reset it if needed.
-Lee

var
  KbdHook : HHook;
 
//call initially to set numlock on
procedure SetNumLock(Lock : Boolean);
var
  KeyState : TKeyboardState;
begin
  GetKeyboardState(KeyState);
  if Lock and (KeyState[VK_NUMLOCK] = 0) then begin
    //simulate key presses
    Keybd_Event(VK_NUMLOCK, 1, KEYEVENTF_EXTENDEDKEY, 0);
    Keybd_Event(VK_NUMLOCK, 1, KEYEVENTF_EXTENDEDKEY or KEYEVENTF_KEYUP, 0);
  end else if not Lock and (KeyState[VK_NUMLOCK] = 1) then begin
    //simulate key presses
    Keybd_Event(VK_NUMLOCK, 0, KEYEVENTF_EXTENDEDKEY, 0);
    Keybd_Event(VK_NUMLOCK, 0, KEYEVENTF_EXTENDEDKEY or KEYEVENTF_KEYUP, 0);
  end;
end;
 
function KeyboardHookProc(nCode : Integer; wParam : Word; lParam : LongInt) : LongInt;
   stdcall;
begin
  if nCode < 0 then begin
    //If code is less than zero, the hook procedure must pass the message to
    //the CallNextHookEx function without further processing and should
    //return the value returned by CallNextHookEx
    Result := CallNextHookEx(KbdHook, nCode, wParam, lParam);
    Exit;
  end;
  if (wParam = VK_NUMLOCK) then begin
    Result := 0; //stop further processing
    SetNumLock(True); //re-set numlock
  end else
    Result := CallNextHookEx(KbdHook, nCode, wParam, lParam);
end;
 
//do this either in form create/destroy or somewhere similar.
 
procedure TForm1.FormCreate(Sender : TObject);
begin
  //Set the keyboard hook so we can intercept keyboard input
  KbdHook := SetWindowsHookEx(WH_KEYBOARD, @KeyboardHookProc, HInstance,
     GetCurrentThreadId());
end;
 
procedure TForm1.FormDestroy(Sender : TObject);
begin
  //unhook
  UnHookWindowsHookEx(KbdHook);
end;

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Author Comment

by:AuthorwareXtras
ID: 22820043
Does this not have to be done from a DLL so the hook is system wide?
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Accepted Solution

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leei earned 500 total points
ID: 22820326
The example applies to either an application or DLL. You will want to replace the call to GetCurrentThreadId() with 0 if establishing the hook form a DLL, though.
Of course, with a DLL you need to establish and release the windows keyboard hook in some place other than the sample form methods I provided.
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Author Comment

by:AuthorwareXtras
ID: 22822267
In a Delphi app it works fine.

In a DLL,  when I press the numlock key, the numlock indicator flashes on and off repeatedly and the whole machine slows down?
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Author Comment

by:AuthorwareXtras
ID: 22823266
When I change this line:

Result := 0; //stop further processing
to a non zero value (e.g.  Result := 1;)

Then it functions.
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Expert Comment

by:leei
ID: 22825858
My mistake. You are correct - a non zero result should be returned from the keyboard hook to stop further processing.
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