C#窗体没有打开时，就打开一个；如果打开了，再次双击exe后，已打开的窗体就前置；如果窗体已经打开并最小化，就还原窗体并前置！

windowns窗体应用项目：

using System.Threading;
using System.Diagnostics;
using System.Runtime.InteropServices;
        //以下所有代码放在窗体的Program.cs里面
        [DllImport("user32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);
        [DllImport("user32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int nCmdShow);
        [DllImport("user32.dll")]
        private static extern bool IsIconic(IntPtr hWnd);
        private const int SW_RESTORE = 9;
        /// <summary>
        /// c# 进程窗口显示并前置
        /// </summary>
        public static void RaiseOtherProcess()
        {
            Process proc = Process.GetCurrentProcess();
            Process.GetProcesses();
            foreach (Process otherProc in Process.GetProcessesByName(proc.ProcessName))
            {
                //ignore "this" process
                if (proc.Id != otherProc.Id)
                {
                    // Found a "same named process".
                    // Assume it is the one we want brought to the foreground.
                    // Use the Win32 API to bring it to the foreground.
                    IntPtr hWnd = otherProc.MainWindowHandle;
                    if (IsIconic(hWnd))
                    {
                        ShowWindowAsync(hWnd, 9);
                    }
                    SetForegroundWindow(hWnd);
                    break;
                }
            }
        }
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            bool createdNew;
            Mutex instance = new Mutex(true, "Regfrm", out createdNew);
            if (createdNew)
            {
                Application.Run(new Regfrm());
                instance.ReleaseMutex();
                //如果需要窗体重启，并出现问题，将这句改为
                //instance.Dispose();
            }
            else
            {
                RaiseOtherProcess();
                Application.Exit();
            }
        }

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windowns窗体应用插件项目：

        public MainFrm Frm;//这一句放在class里面
        private void Button1_Click(object sender, RibbonControlEventArgs e)
        {
             //以下代码放在插件控件的事件里面
            if (Frm == null || Frm.IsDisposed)
            {
                Frm = new MainFrm();
                //ZwSoft.ZwCAD.ApplicationServices.Application.ShowModelessDialog(Frm);
                Frm.Show();
            }
            else
            {
                if (Frm.WindowState == FormWindowState.Minimized)
                {
                    Frm.WindowState = FormWindowState.Normal;
                }
                Frm.Activate();
            }
            //以上代码放在插件控件的事件里面
        }