public partial class App : Application   {       private static Semaphore singleInstanceWatcher;       private static bool createdNew;        static App()       {           // Ensure other instances of this application are not running.           singleInstanceWatcher = new Semaphore(               0, // Initial count.               1, // Maximum count.               Assembly.GetExecutingAssembly().GetName().Name,               out createdNew);            if (createdNew)           {               // This thread created the kernel object so no other instance               // of this application must be running.               //Application currApp = Application.Current;               //currApp.StartupUri = new Uri("MainWindow.xaml", UriKind.RelativeOrAbsolute);               MainWindow win = new MainWindow();               win.Show();           }           else           {               // This thread opened an existing kernel object with the same               // string name; another instance of this app must be running now.               // Gets a new System.Diagnostics.Process component and the               // associates it with currently active process.               Process current = Process.GetCurrentProcess();                // Enumerate through all the process resources on the share               // local computer that the specified process name.               foreach (Process process in                    Process.GetProcessesByName(current.ProcessName))               {                   if (process.Id != current.Id)                   {                       NativeMethods.SetForegroundWindow(                           process.MainWindowHandle);                       NativeMethods.ShowWindow(process.MainWindowHandle,                           WindowShowStyle.Restore);                       break;                   }               }                // Terminate this process and gives the underlying operating                // system the specified exit code.               Environment.Exit(-2);           }  // 如果代码在Application_Startup中 如下       // private Semaphore singleInstanceWatcher;         // private bool createdNew;                      //private void Application_Startup(object sender, StartupEventArgs e)           //{           //    singleInstanceWatcher = new Semaphore(           //   0, // Initial count.           //   1, // Maximum count.           //   Assembly.GetExecutingAssembly().GetName().Name,           //   out createdNew);           //    if (createdNew)           //    {           //        // This thread created the kernel object so no other instance           //        // of this application must be running.           //        Application currApp = Application.Current;           //        currApp.StartupUri = new Uri("MainWindow.xaml", UriKind.RelativeOrAbsolute);           //    }           //    else           //    {           //        //MessageBox.Show("系统已经运行");           //        Process current = Process.GetCurrentProcess();           //        foreach (Process process in           //             Process.GetProcessesByName(current.ProcessName))           //        {           //            if (process.Id != current.Id)           //            {                                      //                NativeMethods.SetForegroundWindow(           //                    process.MainWindowHandle);           //                NativeMethods.ShowWindow(process.MainWindowHandle,           //                    WindowShowStyle.Restore);           //                break;           //            }           //        }           //        Environment.Exit(-2);           //    }                 }   }       internal enum WindowShowStyle : uint       {           /// <summary>Hides the window and activates another window.</summary>           /// <remarks>See SW_HIDE</remarks>           Hide = 0,           /// <summary>Activates and displays a window. If the window ..           /// or maximized, the system restores it to its original size and           /// position. An application should specify this flag when displaying           /// the window for the first time.</summary>           /// <remarks>See SW_SHOWNORMAL</remarks>           ShowNormal = 1,           /// <summary>Activates the window and displays it ..</summary>           /// <remarks>See SW_SHOWMINIMIZED</remarks>           ShowMinimized = 2,           /// <summary>Activates the window and displays it ..</summary>           /// <remarks>See SW_SHOWMAXIMIZED</remarks>           ShowMaximized = 3,           /// <summary>Maximizes the specified window.</summary>           /// <remarks>See SW_MAXIMIZE</remarks>           Maximize = 3,           /// <summary>Displays a window in its most recent size and position.           /// This value is similar to "ShowNormal", except the window is not           /// actived.</summary>           /// <remarks>See SW_SHOWNOACTIVATE</remarks>           ShowNormalNoActivate = 4,           /// <summary>Activates the window and displays it in its current size           /// and position.</summary>           /// <remarks>See SW_SHOW</remarks>           Show = 5,           /// <summary>Minimizes the specified window and activates the next           /// top-level window in the Z order.</summary>           /// <remarks>See SW_MINIMIZE</remarks>           Minimize = 6,           /// <summary>Displays the window as a minimized window. This value is           /// similar to "ShowMinimized", except the window ..</summary>           /// <remarks>See SW_SHOWMINNOACTIVE</remarks>           ShowMinNoActivate = 7,           /// <summary>Displays the window in its current size and position. This           /// value is similar to "Show", except the window ..</summary>           /// <remarks>See SW_SHOWNA</remarks>           ShowNoActivate = 8,           /// <summary>Activates and displays the window. If the window is           /// minimized or maximized, the system restores it to its original size           /// and position. An application should specify this flag ..           /// a minimized window.</summary>           /// <remarks>See SW_RESTORE</remarks>           Restore = 9,           /// <summary>Sets the show state based on the SW_ value specified ..           /// STARTUPINFO structure passed to the CreateProcess function by the           /// program that started the application.</summary>           /// <remarks>See SW_SHOWDEFAULT</remarks>           ShowDefault = 10,           /// <summary>Windows 2000/XP: Minimizes a window, even if the thread           /// that owns the window is hung. This flag should only be used when           /// minimizing windows from a different thread.</summary>           /// <remarks>See SW_FORCEMINIMIZE</remarks>           ForceMinimized = 11       }       static class NativeMethods       {           /// <summary>           /// Brings the thread that created the specified window into the           /// foreground and activates the window. Keyboard input is directed           /// to the window, and various visual cues are changed for the user.           /// The system assigns a slightly higher priority to the thread that           /// created the foreground window than it does to other threads.           /// </summary>           /// <param name="hWnd">A handle to the window that should be           /// activated and brought to the foreground.           /// </param>           /// <returns>If the window was brought to the foreground, the           /// return value is nonzero. </returns>           [DllImport("user32.dll")]           internal static extern bool SetForegroundWindow(IntPtr hWnd);            /// <summary>Shows a Window</summary>           /// <remarks>           /// <para>To perform certain special effects when showing or hiding a           /// window, use AnimateWindow.</para>           /// <para>The first time an application calls ShowWindow, it should use           /// the WinMain function‘s nCmdShow parameter as its nCmdShow ..           /// Subsequent calls to ShowWindow must use one of the values in the           /// given list, instead of the one specified by the WinMain function‘s           /// nCmdShow parameter.</para>           /// <para>As noted in the discussion of the nCmdShow parameter, the           /// nCmdShow value is ignored in the first call to ShowWindow if the           /// program that launched the application specifies startup information           /// in the structure. In this case, ShowWindow uses the information           /// specified in the STARTUPINFO structure to show the window. On           /// subsequent calls, the application must call ShowWindow with ..           /// set to SW_SHOWDEFAULT to use the startup information provided by ..           /// program that launched the application. This behavior is designed ..           /// the following situations: </para>           /// <list type="">           ///    <item>Applications create their main window by calling ..           ///    with the WS_VISIBLE flag set. </item>           ///    <item>Applications create their main window by calling ..           ///    with the WS_VISIBLE flag cleared, and later call ShowWindow ..           ///    SW_SHOW flag set to make it visible.</item>           /// </list></remarks>           /// <param name="hWnd">Handle to the window.</param>           /// <param name="nCmdShow">Specifies how the window is to be shown.           /// This parameter is ignored the first time an application calls           /// ShowWindow, if the program that launched the application provides a           /// STARTUPINFO structure. Otherwise, the first time ShowWindow .. ,           /// the value should be the value obtained by the WinMain function ..           /// nCmdShow parameter. In subsequent calls, this parameter ..           /// the WindowShowStyle members.</param>           /// <returns>           /// If the window was previously visible, the return value is nonzero.           /// If the window was previously hidden, the return value is zero.           /// </returns>           [DllImport("user32.dll")]           internal static extern bool ShowWindow(IntPtr hWnd,               WindowShowStyle nCmdShow);       }