求救！获取线程ID的问题
今天遇到一个问题，就是我想获得线程ID，但是GetWindowThreadProcessId()的第二个参数我该怎么写？我写null但是有错误，谢谢解答！
[解决办法]
refer:

[DllImport("User32.dll", CharSet = CharSet.Auto)]
        public static extern int GetWindowThreadProcessId(IntPtr hwnd, out int ID);

        [DllImport("user32.dll", EntryPoint = "FindWindow")]
        private extern static IntPtr FindWindow(string lpClassName, string lpWindowName);

        static void Main(string[] args)
        {
            IntPtr hwnd = FindWindow(null, "计算器");
            if (hwnd != IntPtr.Zero)
            {
                int calcID;
                //获取进程ID  
                GetWindowThreadProcessId(hwnd, out calcID);
            }
        }