WPF Single Instance Application

The Problem

The question this post solves is how to enforce that your WPF application has only one instance?

Solution Source

The solution is based on code found in some WPF reference applications which Microsoft will soon (?) release. I didn't write it, but I used it several times and it’s the best solution I've found to date, so I'd hate to see it unpublished.

Solution Advantages

So, what are the advantages of this solution? After all, it’s not the first time someone posts a solution for this problem.

Well, the most important advantage is that it works.
No glitches. No special cases. No inherent race conditions.
It simply works.

Second, it’s easily used. In the next section, I'll show you exactly how to use it.

Third, there are no constraints on your WPF application. Specifically, your main application / window class doesn't have to inherit some base class for this to work.

And at last, you don't need to be dependent on any VB DLL.
I say this because one of the popular solutions for this problem requires you to add a reference to a (WinForms related!) Visual Basic DLL, which may feel strange for some people.

As a bonus, the solution provides to the first instance the parameters of the second instance when run. This is very useful if you want to integrate you application with the Windows 7 taskbar.

Solution Details

So, let’s see how to make your WPF application having just one instance.

Step 1: Add the file SingleInstance.cs to your project.

Step 2: Add a reference to your project: System.Runtime.Remoting.

Step 3: Have your application class implement ISingleInstanceApp (defined in SingleInstance.cs).

The only method in this interface is:

bool SignalExternalCommandLineArgs(IList<string> args)

This method is called when a second instance of your application tries to run. It has an args parameter which is the same as the command line arguments passed to the second instance.

Step 4: Define your own Main function that uses the single instance class.

Your App class should now be similar to this:

/// <summary>
/// Interaction logic for App.xaml
/// </summary>
public partial class App : Application, ISingleInstanceApp
{
    private const string Unique = "My_Unique_Application_String";
    [STAThread]
    public static void Main()
    {
        if (SingleInstance<App>.InitializeAsFirstInstance(Unique))
        {
            var application = new App();
            application.InitializeComponent();
            application.Run();
            // Allow single instance code to perform cleanup operations
            SingleInstance<App>.Cleanup();
        }
    }
    #region ISingleInstanceApp Members
    public bool SignalExternalCommandLineArgs(IList<string> args)
    {
        // handle command line arguments of second instance
        // ...
        return true;
    }
    #endregion
}

Step 5: Set new main entry point.

Select Project Properties –> Application and set “Startup object” to your App class name instead of “(Not Set)”.

Step 6: Cancel the default WPF main function.

Right click on App.xaml, Properties, set Build Action to "Page" instead of "Application Definition".

Solution Inner Works

I can summarize it as: Mutex and Remoting, done right.
If you want to know more details, just have a look at the code.

You can find a WPF sample application here.

That’s it for now,
Arik Poznanski.