//定义结构体
internal struct LASTINPUTINFO
{
public uint cbSize;
public uint dwTime;
}

//引入系统API
[DllImport("User32.dll")]
private static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);

/// <summary>
/// get the system idle time
/// </summary>
/// <returns></returns>
public long getIdleTick()
{
LASTINPUTINFO vLastInputInfo = new LASTINPUTINFO();
vLastInputInfo.cbSize = (uint)Marshal.SizeOf(vLastInputInfo);
if (!GetLastInputInfo(ref　 vLastInputInfo)) return 0;
return (Environment.TickCount - (long)vLastInputInfo.dwTime)/1000;
}

应用时最好单独开启一个线程来计算  不要使用UI线程

private bool autoClose = false;
private bool isRun = true;
Thread monitorSystemFree = null;

Thread monitorSystemFree = new Thread(StartAutoClose);
monitorSystemFree.IsBackground = true;
monitorSystemFree.Start();

/// <summary>
/// auto to close window
/// </summary>
private void StartAutoClose()
{
if (AutoClose)
{
while (this.isRun)
{
//Get System run ticks
long idleTicks = getIdleTick();
// auto close time out : 15 s (1000 is to let the result close to 15 s)
if (idleTicks >= 15)
{
this.isRun = false;

this.Dispatcher.Invoke(new Action(() =>
{
this.Close();
}), null);
}
}
}
}

获取系统空闲时间