Implement binding to attached properties about winrtmultibinding

[Ingerdev]

May be this code will help you, i updated old SLMultibinding to uwp and this part looks like this:
``
...
if (relay.TargetProperty.Contains("."))
{
// split to find the type and property name
string[] parts = relay.TargetProperty.Split('.');
targetType = getXamlUiControlType(parts[0]);
targetProperty = parts[1];
}
else
{
targetType = targetElement.GetType();
targetProperty = relay.TargetProperty;
}

            var sourceFields = targetType.GetTypeInfo().DeclaredProperties;//; targetType.GetFields(DpFlags);//.OrderByDescending(x => x.Name).ToArray();                
            var targetDependencyPropertyField =
                sourceFields.First(i => i.Name == targetProperty + "Property");

            DependencyProperty targetDependencyProperty =
                targetDependencyPropertyField.GetValue(null) as DependencyProperty;

            // bind the ConvertedValue of our MultiBinding instance to the target property
            // of our targetElement
            Windows.UI.Xaml.Data.Binding binding = new Windows.UI.Xaml.Data.Binding()
            {
                Path = new PropertyPath("ConvertedValue"),
                Source = relay,
                Mode = relay.Mode
            };
            Debug.WriteLine(targetElement.ToString());
            targetElement.SetBinding(targetDependencyProperty, binding);
        }
    }

    private string _xamlUiFullqualifiedAssemblyName = string.Empty;
    private Type getXamlUiControlType(string shortTypename)
    {
        if (string.IsNullOrEmpty(_xamlUiFullqualifiedAssemblyName))
        {
            _xamlUiFullqualifiedAssemblyName = generateXamlUiFullqualifiedAssemblyname();
        }
        return Type.GetType(string.Format("Windows.UI.Xaml.Controls.{0}{1}",shortTypename,_xamlUiFullqualifiedAssemblyName));         
    }

    private string generateXamlUiFullqualifiedAssemblyname()
    {
        //", Windows, Version = 255.255.255.255, Culture = neutral, PublicKeyToken = null, ContentType = WindowsRuntime");
        var typename = typeof(Windows.UI.Xaml.Controls.Canvas).GetTypeInfo().AssemblyQualifiedName;
        return typename.Substring(typename.IndexOf(','));
    }

[Ingerdev]

Note that each type that will use multibinding need a record in default.rd.xml like

  <Type Name="Windows.UI.Xaml.Controls.Canvas"  Dynamic="Required All" />
    <Type Name="Windows.UI.Xaml.Controls.TextBlock"  Dynamic="Required All" />