C# Alternative Class/Library To Using PostMessage (user32.dll) To Send Keystrokes?

Question

I have an application that needs to send keystrokes to another process. I have had limited success using the PostMessage function defined in user32.dll. I can send normal keystrokes but I cannot send keystrokes with modifiers (eg. ALT, CTRL).

Let me show you what I currently have...

Constants:

public const ushort WM_KEYDOWN = 0x0100;
public const ushort WM_KEYUP = 0x0101;
public const ushort WM_SYSKEYDOWN = 0x0104;
public const ushort WM_SYSKEYUP = 0x0105;
public const ushort WM_SYSCHAR = 0x0106;
public const ushort VK_MENU = 0x12; // ALT key

DLL Import:

[DllImport("user32.dll")]
public static extern int PostMessage(IntPtr hWnd, int Msg, uint wParam, int lParam);

Method to send a single key:

public static void PostKey(ushort key, IntPtr hWnd)
{
    SetActiveWindow(hWnd);
    PostMessage(hWnd, WM_KEYDOWN, key, 0);
    PostMessage(hWnd, WM_KEYUP, key, 0);
}

Method to send a single key along with ALT (doesn't work):

public static void PostKeyWithAlt(ushort key, IntPtr hWnd)
{
    SetActiveWindow(hWnd);
    PostMessage(hWnd, WM_SYSKEYDOWN, VK_MENU, 0);
    PostMessage(hWnd, WM_SYSKEYDOWN, key, 0);
    PostMessage(hWnd, WM_SYSKEYUP, key, 0);
    PostMessage(hWnd, WM_KEYUP, VK_MENU, 0);
}

PostKey works flawlessly. I am able to connect to another process, get the main window handle, and send any arbitrary keystroke I want.

The problem arises when I try to use PostKeyWithAlt. This just doesn't seem to work. (The key gets sent but not the ALT so it's basically the same as PostKey.) I have Googled and tried so many combinations of method calls I am quite frankly ready to call it quits. :P

So, in desperation, I used the Visual Studio Spy++ tool to watch what happens when I hit ALT+F1 in the target application. Here is the result of the message capture:

Spy++ Log:

<00001> 00080228 P WM_SYSKEYDOWN nVirtKey:VK_MENU cRepeat:1 ScanCode:38 fExtended:0 fAltDown:1 fRepeat:0 fUp:0
<00002> 00080228 P WM_SYSKEYDOWN nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:0 fUp:0
<00003> 00080228 P WM_SYSKEYUP nVirtKey:VK_F1 cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:1 fUp:1
<00004> 00080228 P WM_KEYUP nVirtKey:VK_MENU cRepeat:1 ScanCode:38 fExtended:0 fAltDown:0 fRepeat:1 fUp:1

As you can see, the sequence is exactly the same sequence as my PostKeyWithAlt function. So I would think that it would work but it's not.

There must be something else going on here. The only thing I can think of is the 4th parameter to the PostMessage function. According to the Microsoft documentation this 4th parameter is defined as LPARAM lParam (see below for the full function definition of PostMessage).

Perhaps I need to be setting lParam to something other than 0? If so, what value(s) do I need to set? Can I use the Spy++ log to figure it out? If so, it's beyond me how to interpret anything past _nVirtKey:VK_MENU_ in those log messages:

<00001>.... cRepeat:1 ScanCode:38 fExtended:0 fAltDown:1 fRepeat:0 fUp:0
<00002>.... cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:0 fUp:0
<00003>.... cRepeat:1 ScanCode:3B fExtended:0 fAltDown:1 fRepeat:1 fUp:1
<00004>.... cRepeat:1 ScanCode:38 fExtended:0 fAltDown:0 fRepeat:1 fUp:1

If I could translate the log messages above to the correct lParam argument it just might work.

Any help/suggestions on how to do so would be greatly appreciated.

Alternatively, is there is another more elegant way to send keystrokes to a process? Perhaps a C# library? I know about the SendKeys class but that class doesn't send keystrokes to other applications.

Thanks!

PostMessage Function Definition:

BOOL WINAPI PostMessage(
  _In_opt_  HWND hWnd,
  _In_      UINT Msg,
  _In_      WPARAM wParam,
  _In_      LPARAM lParam
);