[System.Runtime.InteropServices.DllImport("user32.dll", CharSet =System.Runtime.InteropServices.CharSet.Auto, ExactSpelling =true)]
  public static extern IntPtrGetForegroundWindow();   //获得当前活动窗体的句柄

[System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint= "SetForegroundWindow")]
  public static extern boolSetForegroundWindow(IntPtr hWnd);//设置此窗体句柄的窗体为活动窗体

GetForegroundWindow()，此函数返回一个当前活动窗体的句柄
SetForegroundWindow(this. Handle  )设置自己为活动窗体