Emulating keystrokes in Windows

One of the core functions in Shoot, a voice command program I wrote a couple of years ago, is the ability to emulate key presses programmatically.

There is a fairly straightforward way to do this in Windows 2000/XP, via the SendInput Win32 function. In Windows 98/ME things are somewhat more complicated: SendInput inserts the keystrokes in the Windows event queue, but DirectInput bypasses it and hooks directly into the keyboard driver.

As a result, any keystroke sent via SendInput is completely ignored by DirectInput applications. Pretty much every modern Windows-based game uses DirectInput, the main target for Shoot. The solution is to implement a Virtual Device Driver (VxD) that inserts the keystrokes directly into the keyboard driver buffer, and that is what I ended up doing for Shoot.

Here's how to use the SendInput function from C#. I stripped all the Win98/ME-related code. Not only would it make the code below significantly more complex, but it's probably useless by now. Who's using Win 98, anyway? Not even Microsoft supports it anymore :)

[StructLayout(LayoutKind.Sequential)]
private struct KEYBOARD_INPUT {
public uint type;
public ushort vk;
public ushort scanCode;
public uint flags;
public uint time;
public uint extrainfo;
public uint padding1;
public uint padding2;
}

[DllImport("User32.dll")]
private static extern uint SendInput(uint numberOfInputs, [MarshalAs(UnmanagedType.LPArray, SizeConst=1)] KEYBOARD_INPUT[] input, int structSize);

void press(int scanCode)
{
  sendKey(scanCode, true);
}

void release(int scanCode)
{
  sendKey(scanCode, false);
}

private void sendKey(int scanCode, bool press)
{
   KEYBOARD_INPUT[] input = new KEYBOARD_INPUT[1];
   input[0] = new KEYBOARD_INPUT();
   input[0].type = INPUT_KEYBOARD;
   input[0].flags = KEY_SCANCODE;

   if ((scanCode & 0xFF00) == 0xE000) { // extended key?
     input[0].flags |= KEY_EXTENDED;
   }

   if (press) { // press?
     input[0].scanCode = (ushort) (scanCode & 0xFF);
   }
   else { // release?
     input[0].scanCode = scanCode;
     input[0].flags |= KEY_UP;
   }

   uint result = SendInput(1, input, Marshal.SizeOf(input[0]));

   if (result != 1) {
     throw new Exception("Could not send key: " + scanCode);
   }
}