Sending specific keys on the Numpad like +, -, / or Enter (simulating a keypress)

Question

I am working on a project where it is necessary to simulate key-presses to cause specific behaviours in a different application.

All is running well and fine, using

Answer 1

thanks to andreas for providing the beginning of a solution. here's a more complete version:

[DllImport("user32.dll")]
private static extern IntPtr PostMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();
[DllImport("user32.dll")]
private static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);
[DllImport("user32.dll")]
private static extern bool GetGUIThreadInfo(uint idThread, out GUITHREADINFO lpgui);

public struct GUITHREADINFO
{
    public int cbSize;
    public int flags;
    public int hwndActive;
    public int hwndFocus;
    public int hwndCapture;
    public int hwndMenuOwner;
    public int hwndMoveSize;
    public int hwndCaret;
    public System.Drawing.Rectangle rcCaret;
}

private void sendNumpadEnter()
{
    bool keyDown = true; // true = down, false = up
    const uint WM_KEYDOWN = 0x0100;
    const uint WM_KEYUP = 0x0101;
    const int VK_RETURN = 0x0D;

    IntPtr handle = IntPtr.Zero;
    // Obtain the handle of the foreground window (active window and focus window are only relative to our own thread!!)
    IntPtr foreGroundWindow = GetForegroundWindow();
    // now get process id of foreground window
    uint processID;
    uint threadID = GetWindowThreadProcessId(foreGroundWindow, out processID);
    if (processID != 0)
    {
        // now get element with (keyboard) focus from process
        GUITHREADINFO threadInfo = new GUITHREADINFO();
        threadInfo.cbSize = Marshal.SizeOf(threadInfo);
        GetGUIThreadInfo(threadID, out threadInfo);
        handle = (IntPtr)threadInfo.hwndFocus;
    }

    int lParam = 1 << 24; // this specifies NumPad key (extended key)
    lParam |= (keyDown) ? 0 : (1 << 30 | 1 << 31); // mark key as pressed if we use keyup message

    PostMessage(handle, (keyDown) ? WM_KEYDOWN : WM_KEYUP, VK_RETURN, lParam); // send enter
}