C# 只允许运行一个程序实例

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;//使用DllImport的必须。
using System.Diagnostics;//引入Process 类

namespace 命名空间
{
    static class Program
    {

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

        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Process instance = GetRunningInstance();
            if (instance == null)
            {
                Application.EnableVisualStyles();
                Application.SetCompatibleTextRenderingDefault(false);
                Application.Run(new frm_Main());//在这启动主窗体。
            }
            else
            {
                HandleRunningInstance(instance);
            }
        }
        /// <summary>
        /// 获取当前是否具有相同进程。
        /// </summary>
        /// <returns></returns>
        public static Process GetRunningInstance()
        {
            Process current = Process.GetCurrentProcess();
            Process[] processes = Process.GetProcessesByName(current.ProcessName);
            //遍历正在有相同名字运行的例程
            foreach (Process process in processes)
            {
                //忽略现有的例程
                if (process.Id != current.Id)
                    //确保例程从EXE文件运行
                    if ( System.Reflection.Assembly.GetExecutingAssembly().Location.Replace("/" , "\\") == current.MainModule.FileName )
                        return process;
            }
            return null;
        }
        /// <summary>
        /// 激活原有的进程。
        /// </summary>
        /// <param name="instance"></param>
        public static void HandleRunningInstance(Process instance)
        {
            ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL);
            SetForegroundWindow(instance.MainWindowHandle);
        }
    }
}