C# 实现程序只启动一次（多次运行激活第一个实例,使其获得焦点,并在最前端显示）

using System.Runtime.InteropServices;
using System.Diagnostics;
using System.Reflection;
//*****************************************************
  static class Program
    {
        /// <summary>
        /// 该函数设置由不同线程产生的窗口的显示状态。
        /// </summary>
        /// <param name="hWnd">窗口句柄</param>
        /// <param name="cmdShow">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。</param>
        /// <returns>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。</returns>
        [DllImport("User32.dll")]
        private static extern bool ShowWindowAsync(IntPtr hWnd, int cmdShow);
        /// <summary>
        /// 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。
        /// </summary>
        /// <param name="hWnd">将被激活并被调入前台的窗口句柄。</param>
        /// <returns>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。</returns>
        [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);
            Process instance = RunningInstance();
            if (instance == null)
            {
                Form1 frm = new Form1();
                Application.Run(new Form1());
            }
            else
            {
                HandleRunningInstance(instance);
            }

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

        /// <summary>
        /// 显示已运行的程序。
        /// </summary>
        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
        }
    }

实现让程序只能打开一个实例(其他方法）

//=====创建互斥体法：=====
bool blnIsRunning;
Mutex mutexApp = new Mutex(false, Assembly.GetExecutingAssembly().FullName, out   blnIsRunning);
if (!blnIsRunning)
{
    MessageBox.Show("程序已经运行！", "提示",
    MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
    return;
}   

//保证同时只有一个客户端在运行
System.Threading.Mutex mutexMyapplication = new System.Threading.Mutex(false, "OnePorcess.exe");
if (!mutexMyapplication.WaitOne(100, false))
{
    MessageBox.Show("程序" + Application.ProductName + "已经运行！", Application.ProductName,
    MessageBoxButtons.OK, MessageBoxIcon.Error);
    return;
}

//=====判断进程法：(修改程序名字后依然能执行)=====
Process current = Process.GetCurrentProcess();
Process[] processes = Process.GetProcessesByName(current.ProcessName);
foreach (Process process in processes)
{
    if (process.Id != current.Id)
    {
        if (process.MainModule.FileName
        == current.MainModule.FileName)
        {
            MessageBox.Show("程序已经运行！", Application.ProductName,
            MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
            return;
        }
    }
} 

出处：http://www.soaspx.com/dotnet/csharp/csharp_20130319_10168.html