核心代码是调用 Windows API GetLastInputInfo() ：

        [StructLayout(LayoutKind.Sequential)]
        private struct LASTINPUTINFO
        {
            public uint cbSize;
            public uint dwTime;
        }
        [DllImport("user32.dll")]
        private static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);
        public static uint GetIdleTime()
        {
            LASTINPUTINFO lastInputInfo = new LASTINPUTINFO();
            lastInputInfo.cbSize = (uint)Marshal.SizeOf(lastInputInfo);

            if (GetLastInputInfo(ref lastInputInfo))
            {
                uint idleTime = (uint)Environment.TickCount - lastInputInfo.dwTime;
                return idleTime / 1000; // Convert milliseco