Dependency Property + UserControl + ViewModel

Question

Given I have this UserControl:

public class MyStringUserControl : UserControl
{
    public string MyString
    {
        get { return (string)GetValue(MyStringProperty); }
        set { SetValue(MyStringProperty, value); }
    }

    public static readonly DependencyProperty MyStringProperty =
        DependencyProperty.Register("MyString", typeof(string), typeof(MyStringUserControl),
            new FrameworkPropertyMetadata(null));
}

And this ViewModel:

public class MyStringViewModel
{
    public string MyString { get; set; }
}

Now I use the MyStringUserControl in another View like this:

<controls:MyStringUserControl MyString="{Binding SomeStringProperty} />

I'am looking for an elegant way to bind this string back to the MyStringViewModel.
I also don't feel comfortable with the fact that I have to duplicate every property in the UserControl code behind and ViewModel. Is there a better way to do this?

Edit #1:

The reason I want to do this is because of unit testing (creating a UserControl takes very long even without InitializeComponent)

Answer 1

If I was intent on doing this, I would use DependencyProperty's PropertyChanged event handler to set my ViewModel's property, and my ViewModel's PropertyChanged event handler to set my DependencyProperty. Having said that I've never had a reason to go down this particular road.

private SomeViewModel _viewModel;

public static readonly DependencyProperty MyStringProperty = DependencyProperty.Register("MyString", typeof(string), typeof(MyStringUserControl), new PropertyMetadata(OnMyStringChanged));

public MyStringUserControl()
{
    InitializeComponent();

    _viewModel = new SomeViewModel();
    _viewModel.PropertyChanged += OnViewModelPropertyChanged;

    this.DataContext = _viewModel;
}

private static void OnMyStringChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    ((MyStringUserControl)d).OnMyStringChanged(e.NewValue);
}

private void OnMyStringChanged(string newValue)
{
    _viewModel.SomeProperty = newValue;
}

private void OnViewModelPropertyChanged(object sender, PropertyChangedEventArgs e)
{
    switch (e.PropertyName)
    {
        case "SomeProperty":
            SetValue(MyStringProperty, _viewModel.SomeProperty);
            break;
        default:
            break;
    }
}

Answer 2

There is absolutely no point in duplicating your properties. Using MVVM does not mean that you need to have a view model for every UserControl . When I use a UserControl as a part of a view, I rarely use a separate view model for it. Sometimes, I'll just use the DependencyProperty s in the UserControl code behind, while other times I'll just data bind to the parent view model directly. It all depends on what you want to do with the data.