Question

Given a handle of a window, how can I close the window by using the window handle?

Answer 1

The easiest way is to use PInvoke and do a SendMessage with WM_CLOSE.

[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern IntPtr SendMessage(IntPtr hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);

private const UInt32 WM_CLOSE          = 0x0010;

void CloseWindow(IntPtr hwnd) {
  SendMessage(hwnd, WM_CLOSE, IntPtr.Zero, IntPtr.Zero);
}