The SingleInstanceApp class provides a convenient way to create a single instance application. When your app is launched, it will only allow one instance to run at a time. If another instance of the app is launched, the app will redirect to the already running instance.

1. Disable auto-generated Program code

We need to check for redirection as early as possible, before creating any windows. To do this, we must define the symbol DISABLE_XAML_GENERATED_MAIN in the project file.

Add the following XML to the project file:

<PropertyGroup>
  <DefineConstants>$(DefineConstants);DISABLE_XAML_GENERATED_MAIN</DefineConstants>
</PropertyGroup>

Adding the DISABLE_XAML_GENERATED_MAIN symbol will disable the auto-generated Program code for your project.

2. Define a Program class with a Main method

Simple

A customized Program.cs file which inherited from SingleInstanceApp must be created, and call the Run method in your Main function.

public class Program : SingleInstanceApp
{
    [STAThread]
    static int Main(string[] args)
    {
        return Run(args, "MySingleInstanceApp", () => new Program(), () => new App());
    }

    protected override void OnActivated(AppActivationArguments args)
    {
        WindowActivationService.Activate();
    }
}

3. Register SingleInstanceWindowActivator in OnLuanched Method

Register your mainwindow in App.xaml.cs before Activating your MainWindow.

protected override void OnLaunched(Microsoft.UI.Xaml.LaunchActivatedEventArgs args)
{
    MainWindow = new MainWindow();

    SingleInstanceWindowActivator.Register(MainWindow);

    MainWindow.Activate();
}

Demo

you can run demo and see this feature.