Forcing Single Instance for WPF Apps

Sometimes you may not want to allow multiple instances for your WPF apps. You can use a Mutex to accomplish this. Credit goes to this StackOverflow question: "What is the correct way to create a single instance application?" for mashing these ideas together.

public partial class App : Application
{
    [DllImport("user32.dll")]
    private static extern bool ShowWindow(IntPtr hWnd, int cmdShow);
    private const int SW_MAXIMIZE = 3;
    private const int SW_SHOWNORMAL = 1;

    private static Mutex singleInstanceMutex = new Mutex(true, "AnyUniqueStringToYourApp");

    protected override void OnStartup(StartupEventArgs e) {
        base.OnStartup(e);
        if (singleInstanceMutex.WaitOne(TimeSpan.Zero, true))
            Program.OnStartup();
        else {
            var procs = Process.GetProcessesByName(Process.GetCurrentProcess().ProcessName);
            foreach (var p in procs.Where(p => p.MainWindowHandle != IntPtr.Zero)) {
                ShowWindow(p.MainWindowHandle, SW_MAXIMIZE);
                    Application.Current.Shutdown();
                    return;
                }
            }   
        }
    }
}

The basic idea is that the startup code checks the mutex and maximizes the application if the app isn't already open. You can view the MSDN docs for ShowWindow.aspx) and pass any of the associated constants to alter the behavior of the window if it is already running.

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-JP

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