Prevent using Dispatcher.Invoke in WPF code

Regarding this:

This works, but this is not the only event and thus makes my code horrible ugly

Yes, your WPF-based code will definitely be extremely horrible unless you understand and embrace The WPF Mentality.

Basically, all interactions between your custom logic (AKA Business logic or Application Logic) and the WPF UI should manifest in the form of Declarative DataBinding as opposed to the traditional imperative approach.

This means that there should be nothing like this:

UserWindow.Visibility = Visibility.Hidden;

anywhere in your code, simply because introducing things like that makes your code dependent on the UI and thus only executable on the UI thread.

Instead, the WPF approach to that would be to declaratively DataBind the Visibility propety of the UI element (IN XAML) to a relevant bool property that you can operate from the outside, like this:

<UserWindow Visibility="{Binding ShowUserWindow, Converter={my:BoolToVisibilityConverter}}">
   <!-- ... -->
</UserWindow>

Then, you would need to create a relevant class that contains the properties the UI is expecting to bind to. This is called a ViewModel.

Notice that in order to properly support Two-Way WPF DataBinding, your ViewModels must Implement the INotifyPropertyChanged interface.

When doing so, it is also convenient to have the PropertyChanged event from that interface marshalled to the UI thread, so that you no longer have to worry about setting the ViewModel's properties by using the Dispatcher.

Therefore our first step is to have all our ViewModels inherit from a class like this:

(taken from this answer):

public class PropertyChangedBase:INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    protected virtual void OnPropertyChanged(string propertyName)
    {
        //Raise the PropertyChanged event on the UI Thread, with the relevant propertyName parameter:
        Application.Current.Dispatcher.BeginInvoke((Action) (() =>
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }));
    }
}

Once we have our Property Change Notification Dispatch to the UI Thread in place, we can proceed to create a relevant ViewModel that suits, in this case, the UserWindow and it's DataBinding expectations:

public class UserViewModel: PropertyChangedBase
{
    private bool _showUserWindow;
    public bool ShowUserWindow
    {
        get {return _showUserWindow; }
        set
        {
            _showUserWindow = value;
            OnPropertyChanged("ShowUserWindow"); //This is important!!!
        }
    }
}

Finally, you would need to set the Window's DataContext to an instance of it's corresponding ViewModel. One simple way to do that is in the Window's constructor:

public UserWindow() //Window's Constructor
{
    InitializeComponent();  //this is required.

    DataContext = new UserViewModel(); //here we set the DataContext
}

As you can see in this example, there is literally no need to manipulate the UI element's properties in procedural code. This is good not only because it resolves the Thread Affinity issues (because now you can set the ShowUserWindow property from any thread), but also because it makes your ViewModels and logic completely decoupled from the UI and thus testable and more scalable.

This same concept applies to EVERYTHING in WPF.

One detail that I need to mention is that I'm making use of a technique of Combining MarkupExtension and IValueConverter in order to reduce the the XAML boilerplate involved in using Converters.

You can read more about that in the link and also the MSDN DataBinding page linked above.

Let me know if you need further details.