Getting the value of a WPF RelativeSource binding in codebehind

Question

I'm trying to translate this XAML code:

<Binding Path="DataContext" RelativeSource="{RelativeSource AncestorType={x:Type UserControl}}" />

Into this C# code:

var binding = new Binding("DataContext")
{
    RelativeSource = new RelativeSource {AncestorType = typeof(UserControl)}
};
var value = PropertyPathHelper.GetValue(binding);

The implementation of my PropertyPathHelper (modified from another thread) class is as follows:

public static class PropertyPathHelper
{
    public static object GetValue(Binding binding)
    {

        BindingOperations.SetBinding(_dummy, Dummy.ValueProperty, binding);
        return _dummy.GetValue(Dummy.ValueProperty);
    }

    private static readonly Dummy _dummy = new Dummy();

    private class Dummy : DependencyObject
    {
        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register("Value", typeof(object), typeof(Dummy), new UIPropertyMetadata(null));
    }
}

Either my Binding declaration isn't right, or my PropertyPathHelper implementation isn't right, but I don't know which, because at runtime, "var value" comes out as null. Even if I pass a non-existent name into the constructor of Binding.

The binding works fine if I do it in XAML, but I have to do it in the codebehind. In case it's not clear, I'm trying to get the actual value of the DataContext of this view's first ancestor, which is of type UserControl.

What am I doing wrong?

Answer 1

I found a much more elegant way of achieving my goals... and it actually works.

First, in the XAML of my view, I added the following attribute:

Tag="{Binding RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"

Then I found I could just grab what I need with the following code in the codebehind:

var v = this.Tag as FrameworkElement;
var vm = v.DataContext as MyViewModel; //The ViewModel of the parent view, not the current one

Thank you for your questions, @Clemens. In a roundabout way, you helped me think about my problem differently!