使用用互斥量(System.Threading.Mutex)：
    同步基元，它只向一个线程授予对共享资源的独占访问权。在程序启动时候，请求一个互斥体，如果能获取对指定互斥的访问权，就职运行一个实例。

实例代码：

    /// <summary>/// App.xaml 的交互逻辑/// </summary>public partial class App : Application{[DllImport("user32.dll ")]//设置窗体置顶private static extern bool SetForegroundWindow(IntPtr hWnd);[DllImport("User32.dll")]public static extern bool ShowWindow(IntPtr hWnd, int nCmdShow);private static Mutex _m = null;private static string currentProcess = System.Reflection.Assembly.GetExecutingAssembly().GetName().Name;/// <summary>/// 只允许运行一个实例/// </summary>/// <param name="mutexName"></param>/// <returns>true:已经存在实例 false:不存在实例</returns>public static bool CheckRunOneInstance(string mutexName = null){bool result = false;bool mutexWasCreated = false;try{bool requestInitialOwnership = true;if (mutexName == null)mutexName = currentProcess;_m = new Mutex(requestInitialOwnership, mutexName, out mutexWasCreated);if (!mutexWasCreated){result = true;}}catch (Exception ex){MessageBox.Show(ex.Message);}return result;}/// <summary>/// 有程序运行，设置焦点/// </summary>private void Focus(){foreach (Process process in Process.GetProcesses()){try{if (process.ProcessName.ToLower() != currentProcess.ToLower())continue;IntPtr hwnd = process.MainWindowHandle;if (hwnd != IntPtr.Zero){SetForegroundWindow(hwnd);ShowWindow(hwnd, 2);}}catch (Exception ex){MessageBox.Show(ex.Message);}}}private void Application_Startup(object sender, StartupEventArgs e){if(CheckRunOneInstance()){Focus();System.Windows.Application.Current.Shutdown();return;}else{new MainWindow().Show();}}}

实例链接：

https://download.csdn.net/download/lvxingzhe3/88668552