WPF mutex for single app instance not working

Question

I'm trying to use the mutex method for only allowing one instance of my app to run. That is - I only want a max of one instance for all users on a machine. I've read through the various other threads on this issue and the solution seems straightforward enough but in testing I can't get my second instance to not run. Here is my code...

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        // check that there is only one instance of the control panel running...
        bool createdNew = true;
        using (Mutex instanceMutex = new Mutex(true, @"Global\ControlPanel", out createdNew))
        {
            if (!createdNew)
            {
                Application.Current.Shutdown();
                return;
            }
        }

        base.OnStartup(e);
    }
}

score 44 · Accepted Answer · answered Mar 21 '11 at 10:24

44

You're also disposing the mutex in the same method, so the mutex only lives for the duration of the method. Store the mutex in a static field, and keep it alive for the duration of your application.

answered Mar 21 '11 at 10:24

Willem van Rumpt

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44

2

Thanks @Willem van Rumpt (and @OJ). That works. I mistakenly thought the Mutex was really something behind the scenes, more at the system level, and the Mutex object instance wasn't important. I've included my new code below for future reference... – flobadob Mar 21 '11 at 11:11
@OJ: Nice to be on the "winning" side for a change :) – Willem van Rumpt Mar 21 '11 at 11:14
Worked for me. :) – Naveen Kumar V Oct 11 '18 at 13:19

score 33 · Answer 2 · answered Mar 21 '11 at 11:13

33

Here is my new code which has the answer provided by @Willem van Rumpt (and @OJ)...

public partial class App : Application
{
    private Mutex _instanceMutex = null;

    protected override void OnStartup(StartupEventArgs e)
    {
        // check that there is only one instance of the control panel running...
        bool createdNew;
        _instanceMutex = new Mutex(true, @"Global\ControlPanel", out createdNew);
        if (!createdNew)
        {
            _instanceMutex = null;
            Application.Current.Shutdown();
            return;
        }

        base.OnStartup(e);
    }

    protected override void OnExit(ExitEventArgs e)
    {          
        if(_instanceMutex != null)
            _instanceMutex.ReleaseMutex();
        base.OnExit(e);
    }
}

answered Mar 21 '11 at 11:13

flobadob

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why wouldn't you put the duplication check in the constructor? should the `App` call `InitializeComponent` when it won't launch? – Maslow May 10 '18 at 13:50
1

When calling ReleaseMutex from OnExit you may get System.ApplicationException: Object synchronization method was called from an unsynchronized block of code. See [Releasing a named mutex created in WPF Application.OnStartUp(): Which thread owns it?](https://stackoverflow.com/questions/32535686/releasing-a-named-mutex-created-in-wpf-application-onstartup-which-thread-own/32535863#32535863) for a discussion of why. – Bill Menees May 27 '20 at 22:04

Vildi · Answer 3 · 2014-07-24T08:40:10.960

As extension sample:

public static class Extension
{
    private static Mutex mutex;

    public static bool IsOneTimeLaunch(this Application application, string uniqueName = null)
    {
        var applicationName = Path.GetFileName(Assembly.GetEntryAssembly().GetName().Name);
        uniqueName = uniqueName ?? string.Format("{0}_{1}_{2}",
            Environment.MachineName,
            Environment.UserName,
            applicationName);

        application.Exit += (sender, e) => mutex.Dispose();
        bool isOneTimeLaunch;
        mutex = new Mutex(true, uniqueName, out isOneTimeLaunch);
        return isOneTimeLaunch;
    }
}

App Class:

    protected override void OnStartup(StartupEventArgs e)
    {
        if (this.IsOneTimeLaunch())
        {
            base.OnStartup(e);
        }
        else
        {
            this.Shutdown();
        }
    }

score 4 · Answer 4 · answered Mar 21 '11 at 10:27

You're destroying the Mutex immediately after you've created it and tested it. You need to keep the Mutex reference alive for lifetime of your application.

Make the Mutex a member/field of your Application class. Release the mutex when your application shuts down.

score 1 · Answer 5 · edited Sep 08 '15 at 11:17

I was told to implement this mutex approach on an already developed WPF application that we had. The work around to the problem with using override of the OnStart() that I found was in

App.g.cs

This file is located in

obj\x86\debug\

and contains the main() function, so you just simply put this piece of code in your main function.

bool isOnlyInstance = false;
Mutex m = new Mutex(true, @"WpfSingleInstanceApplication", out isOnlyInstance);
if (!isOnlyInstance)
{
    MessageBox.Show("Another Instance of the application is already running.", 
                    "Alert", 
                    MessageBoxButton.OK, 
                    MessageBoxImage.Exclamation);
    return;
}
GC.KeepAlive(m);

but for this you need to keep the BUILD ACTION of your app.xaml set to ApplicationDefinition

NOTE: This might not be the best way, since I'm a beginner. (please tell me if there's something I should change)

score 1 · Answer 6 · answered Nov 08 '11 at 23:33

I can suggest a much cleaner approach that also introduce the useful concept of overriding the Main method in WPF application. Also, if using your solution you take a look at the task manager, you will notice that the new instance actually reach the execution state (you can see a new process created in the list of task) and then suddenly close. The approach proposed in the post that follows will avoid this drawback too. http://blog.clauskonrad.net/2011/04/wpf-how-to-make-your-application-single.html

score 0 · Answer 7 · edited Aug 17 '12 at 00:21

0

I did this from this link just add the given class and a single line in you App.Xaml.cs http://wpfsingleinstance.codeplex.com/

public partial class App : Application    
{
  protected override void OnStartup(StartupEventArgs e) 
  {
        WpfSingleInstance.Make(); //added this is the function of that class 
        base.OnStartup(e);
  }
}

edited Aug 17 '12 at 00:21

S3ddi9

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answered Jun 06 '12 at 15:29

dnxit

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WPF mutex for single app instance not working

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