只允许winform打开一个实例，并且让这实例重新获取焦点

  static class Program
    {
        [DllImport("User32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int cmdShow);

        [DllImport("User32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);
        private const int WS_SHOWNORMAL = 1;



        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            System.Diagnostics.Process[] name = System.Diagnostics.Process.GetProcessesByName(System.Diagnostics.Process.GetCurrentProcess().ProcessName);
            if (name.Length > 1)
            {
                MessageBox.Show("应用程序已启动，请检查！如果没启动请重启计算机");
                ShowWindowAsync(name[0].MainWindowHandle, WS_SHOWNORMAL); 
                SetForegroundWindow(name[0].MainWindowHandle);
            }
            else
            {

                Application.Run(new FrmLogin());

            }
        }
    }