Base view model that can be used in multiple windows

MultiWindowViewModelBase.cs

public class MultiWindowViewModelBase : INotifyPropertyChanged
{
    private readonly ConcurrentDictionary<CoreDispatcher, PropertyChangedEventHandler> _dispatchersAndHandlers = new ConcurrentDictionary<CoreDispatcher, PropertyChangedEventHandler>();

    public event PropertyChangedEventHandler PropertyChanged
    {
        add
        {
            var dispatcher = Window.Current.Dispatcher;

            _dispatchersAndHandlers.AddOrUpdate(
                dispatcher,
                (dispatcher2) => value,
                (dispatcher2, handler2) => handler2 + value);
        }
        remove
        {
            var dispatcher = Window.Current.Dispatcher;

            if (_dispatchersAndHandlers.TryRemove(dispatcher, out PropertyChangedEventHandler handler))
            {
                handler -= value;

                if (handler != null)
                {
                    _dispatchersAndHandlers.AddOrUpdate(
                        dispatcher,
                        (dispatcher2) => handler,
                        (dispatcher2, handler2) => handler2 + handler);
                }
            }
        }
    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        var dispatchersAndHandlers = _dispatchersAndHandlers.ToArray();

        var eventArgs = new PropertyChangedEventArgs(propertyName);

        foreach (var dispatcherAndHandler in dispatchersAndHandlers)
        {
            var dispatcher = dispatcherAndHandler.Key;
            var handler = dispatcherAndHandler.Value;

            dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => handler(this, eventArgs));
        }
    }
}