//调用API
 [System.Runtime.InteropServices.DllImport("user32.dll", CharSet = System.Runtime.InteropServices.CharSet.Auto, ExactSpelling = true)]
 public static extern IntPtr GetForegroundWindow(); //获得本窗体的句柄
 [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "SetForegroundWindow")]
 public static extern bool SetForegroundWindow(IntPtr hWnd);//设置此窗体为活动窗体
 //定义变量,句柄类型
 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);
 }
 }