MVVM and the TextBox's SelectedText property

I have a TextBox with a ContextMenu in it. When the user right clicks inside the TextBox and chooses the appropriate MenuItem, I would like to grab the SelectedText in my viewmodel. I have not found a good way to do this the "MVVM" way.

So far I have my appliction utilizing Josh Smith's way of MVVM. I am looking to tranfer over to Cinch. Not sure if the Cinch framework will handle issues like this. Thoughts?

There's no straightforward way to bind SelectedText to a data source, because it's not a DependencyProperty... however, it quite easy to create an attached property that you could bind instead.

Here's a basic implementation :

public static class TextBoxHelper
{

    public static string GetSelectedText(DependencyObject obj)
    {
        return (string)obj.GetValue(SelectedTextProperty);
    }

    public static void SetSelectedText(DependencyObject obj, string value)
    {
        obj.SetValue(SelectedTextProperty, value);
    }

    // Using a DependencyProperty as the backing store for SelectedText.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SelectedTextProperty =
        DependencyProperty.RegisterAttached(
            "SelectedText",
            typeof(string),
            typeof(TextBoxHelper),
            new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, SelectedTextChanged));

    private static void SelectedTextChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
    {
        TextBox tb = obj as TextBox;
        if (tb != null)
        {
            if (e.OldValue == null && e.NewValue != null)
            {
                tb.SelectionChanged += tb_SelectionChanged;
            }
            else if (e.OldValue != null && e.NewValue == null)
            {
                tb.SelectionChanged -= tb_SelectionChanged;
            }

            string newValue = e.NewValue as string;

            if (newValue != null && newValue != tb.SelectedText)
            {
                tb.SelectedText = newValue as string;
            }
        }
    }

    static void tb_SelectionChanged(object sender, RoutedEventArgs e)
    {
        TextBox tb = sender as TextBox;
        if (tb != null)
        {
            SetSelectedText(tb, tb.SelectedText);
        }
    }

}

You can then use it like that in XAML :

<TextBox Text="{Binding Message}" u:TextBoxHelper.SelectedText="{Binding SelectedText}" />