//調用API
 [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto, ExactSpelling = true)]
 public static extern IntPtr GetForegroundWindow(); //獲得本窗體的句柄
 [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "SetForegroundWindow")]
 public static extern bool SetForegroundWindow(IntPtr hWnd);//設置此窗體為活動窗體
 //定義變量,句柄類型
 public IntPtr han;

void Form1_Load(object sender, EventArgs e)
 {
 han = this.Handle;
 timer2.Enabled = true;
 }

private void timer2_Tick(object sender, EventArgs e)
 {
 if (han != GetForegroundWindow())
 {
 SetForegroundWindow(han);
 }
 }