1.新建一个SingleInstance类:

using System;
using System.Collections.Generic;
using System.Text;

using System.Runtime.InteropServices;
using System.Diagnostics;

namespace NetChange
{
    class SingleInstance
    {
        [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;

        public static Process GetRunningInstance()
        {
            Process currentProcess = Process.GetCurrentProcess();
            string currentFileName = currentProcess.MainModule.FileName;
            Process[] processes = Process.GetProcessesByName(currentProcess.ProcessName);
            foreach (Process process in processes)
            {
                if (process.MainModule.FileName == currentFileName)
                {
                    if (process.Id != currentProcess.Id)
                        return process;
                }
            }
            return null;
        }

       public static bool HandleRunningInstance(Process instance)
       {

          ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL);
          return SetForegroundWindow(instance.MainWindowHandle);
       }

      public static bool HandleRunningInstance()
     {
        Process p = GetRunningInstance();
        if (p != null)
        {
                HandleRunningInstance(p);
                return true;
        }
        return false;
     }

    }  
}
2.在program.cs 修改代码如下:

using System;
using System.Collections.Generic;
using System.Windows.Forms;

namespace NetChange
{
    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            //===定义SingleInstance类，实现应用运行唯一性判断==
            if (SingleInstance.HandleRunningInstance() == false)
            {
                Application.Run(new Form1());
            }
            else {
                 Application.Exit();
            }
            //===========
            //Application.Run(new Form1());
        }
    }
}