SendInput vs. keybd_event

Question

MSDN states that keybd_event has been superceded by SendInput. During a rewrite I switched to using SendInput...which was fine except when trying to send an Alt-key combination. On a Win7 64-bit system (haven't tried elsewhere yet), send an Alt-key causes a long delay before the keystroke is apparent in the target application.

Any ideas why? Or what I've done wrong? For now, I've gone back to keybd_event--the second version below.

//Keyboard input from this version appears only after a ~4-5 second
//time lag...
procedure SendAltM;
var
  KeyInputs: array of TInput;
  KeyInputCount: Integer;
  //--------------------------------------------
  procedure KeybdInput(VKey: Byte; Flags: DWORD);
  begin
    Inc(KeyInputCount);
    SetLength(KeyInputs, KeyInputCount);
    KeyInputs[KeyInputCount - 1].Itype := INPUT_KEYBOARD;
    with  KeyInputs[KeyInputCount - 1].ki do
    begin
      wVk := VKey;
      wScan := MapVirtualKey(wVk, 0);
      dwFlags := KEYEVENTF_EXTENDEDKEY;
      dwFlags := Flags or dwFlags;
      time := 0;
      dwExtraInfo := 0;
    end;
  end;
begin
  KeybdInput(VK_MENU, 0);                 // Alt
  KeybdInput(Ord('M'), 0);                 
  KeybdInput(Ord('M'), KEYEVENTF_KEYUP);   
  KeybdInput(VK_MENU, KEYEVENTF_KEYUP);   // Alt
  SendInput(KeyInputCount, KeyInputs[0], SizeOf(KeyInputs[0]));
end;


//Keyboard input from this version appears immediately...
procedure SendAltM;
begin
  keybd_event( VK_MENU, MapVirtualkey( VK_MENU, 0 ), 0, 0);
  keybd_event( Ord('M'), MapVirtualKey( Ord('M'),0), 0, 0);
  keybd_event( Ord('M'), MapVirtualKey( Ord('M'),0), KEYEVENTF_KEYUP, 0);
  keybd_event( VK_MENU, MapVirtualkey( VK_MENU, 0 ), KEYEVENTF_KEYUP, 0);
end;

Answer 1

Problem 1

You did not initialise KeyInputCount. So its value is undefined. Set it to zero before calling KeybdInput the first time. Or just get rid of it and use Length(KeyInputs) instead.

Problem 2

Your setting of dwFlags is incorrect. Don't include KEYEVENTF_EXTENDEDKEY. You did not include it in the code that calls keybd_event, and you should not include it for SendInput.

Corrected code

This version works.

procedure SendAltM;
var
  KeyInputs: array of TInput;
  //--------------------------------------------
  procedure KeybdInput(VKey: Byte; Flags: DWORD);
  begin
    SetLength(KeyInputs, Length(KeyInputs)+1);
    KeyInputs[high(KeyInputs)].Itype := INPUT_KEYBOARD;
    with  KeyInputs[high(KeyInputs)].ki do
    begin
      wVk := VKey;
      wScan := MapVirtualKey(wVk, 0);
      dwFlags := Flags;
    end;
  end;
begin
  KeybdInput(VK_MENU, 0);                 // Alt
  KeybdInput(Ord('M'), 0);
  KeybdInput(Ord('M'), KEYEVENTF_KEYUP);
  KeybdInput(VK_MENU, KEYEVENTF_KEYUP);   // Alt
  SendInput(Length(KeyInputs), KeyInputs[0], SizeOf(KeyInputs[0]));
end;