只允许运行一个实例

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using System.Runtime.InteropServices;
using System.Diagnostics;
using System.Reflection;

namespace SmsSoft
{
    static class Program
    {

        ///

        /// 该函数设置由不同线程产生的窗口的显示状态。
        ///

        /// 窗口句柄
        /// 指定窗口如何显示。查看允许值列表，请查阅ShowWlndow函数的说明部分。
        /// 如果函数原来可见，返回值为非零；如果函数原来被隐藏，返回值为零。
        [DllImport("User32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int cmdShow);
        ///

        /// 该函数将创建指定窗口的线程设置到前台，并且激活该窗口。键盘输入转向该窗口，并为用户改各种可视的记号。系统给创建前台窗口的线程分配的权限稍高于其他线程。
        ///

        /// 将被激活并被调入前台的窗口句柄。
        /// 如果窗口设入了前台，返回值为非零；如果窗口未被设入前台，返回值为零。
        [DllImport("User32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);
        private const int WS_SHOWNORMAL = 1;
        ///

        /// The main entry point for the application.
        ///

        [STAThread]
        static void Main()
        {
            //Application.EnableVisualStyles();
            //Application.SetCompatibleTextRenderingDefault(false);
            //Application.Run(new Form1());
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Process instance = RunningInstance();
            if (instance == null)
            {
                //Form1 frm = new Form1();
                Application.Run(new Form1());
            }
           else
           {
               HandleRunningInstance(instance);
           }
        }
        ///

        /// 获取正在运行的实例，没有运行的实例返回null;
        ///

        public static Process RunningInstance()
        {
            Process current = Process.GetCurrentProcess();
            Process[] processes = Process.GetProcessesByName(current.ProcessName);
            foreach (Process process in processes)
            {
                if (process.Id != current.Id)
                {
                    if (Assembly.GetExecutingAssembly().Location.Replace("/", "//") == current.MainModule.FileName)
                    {
                        return process;
                    }
                }
            }
            return null;
        }

        ///

        /// 显示已运行的程序。
        ///

        public static void HandleRunningInstance(Process instance)
        {
            ShowWindowAsync(instance.MainWindowHandle, WS_SHOWNORMAL); //显示，可以注释掉
            SetForegroundWindow(instance.MainWindowHandle);            //放到前端
        }

    }
}