激活其他在后台的程序，使窗体活动

private static void HandleRunningInstance(Process instance)
{
    MessageBox.Show("该应用系统已经在运行！", "提示信息", MessageBoxButtons.OK, MessageBoxIcon.Information);
    ShowWindowAsync(instance.MainWindowHandle, 1); //调用api函数，正常显示窗口
    SetForegroundWindow(instance.MainWindowHandle); //将窗口放置最前端。
}
[DllImport("User32.dll")]
private static extern bool ShowWindowAsync(System.IntPtr hWnd, int cmdShow);
[DllImport("User32.dll")]
private static extern bool SetForegroundWindow(System.IntPtr hWnd);