App.xaml.cs

using System.Threading;
using System.Windows;

namespace PinBubble;

public partial class App : System.Windows.Application
{
    private Mutex? _singleInstanceMutex;
    private bool _ownsSingleInstanceMutex;

    protected override void OnStartup(StartupEventArgs e)
    {
        _singleInstanceMutex = new Mutex(true, "PinBubble.SingleInstance", out bool isFirstInstance);
        _ownsSingleInstanceMutex = isFirstInstance;
        if (!isFirstInstance)
        {
            System.Windows.MessageBox.Show("PinBubble is already running. Only one instance can run at a time.", 
                                           "PinBubble", 
                                           MessageBoxButton.OK, 
                                           MessageBoxImage.Error);
            Shutdown();
            return;
        }

        base.OnStartup(e);
    }

    protected override void OnExit(ExitEventArgs e)
    {
        if (_ownsSingleInstanceMutex)
            _singleInstanceMutex?.ReleaseMutex();

        _singleInstanceMutex?.Dispose();
        _singleInstanceMutex = null;
        _ownsSingleInstanceMutex = false;
        base.OnExit(e);
    }
}