ElementName binding in resources does not work when used with DynamicResource, but works with StaticResource

In WPF UserControl, I bind an Color

item property SolidColorBrush

in Resources to a dependency property in UserControl. If I use SolidColorBrush

with StaticResource

, everything works fine, however, if I try to use DynamicResource

, the communication fails with an error. While I can work around the problem by using at least one StaticResource to reference SolidColorBrush

, I want to know why it behaves in such a way as to improve my understanding of WPF.

Here's UserControl xaml, codebehind and error ...

<UserControl x:Class="TestUserControl"
                x:Name="MyUserControl"
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <UserControl.Resources>
        <SolidColorBrush x:Key="BrushKey" Color="{Binding ElementName=MyUserControl,Path=TestProp}"></SolidColorBrush>
    </UserControl.Resources>
    <StackPanel>
        <TextBlock Background="{DynamicResource BrushKey}">Foo</TextBlock>
        <!-- Uncommenting this makes everything work, including the DynamicResource reference. -->
        <!--<TextBlock Background="{StaticResource BrushKey}">Bar</TextBlock>-->
    </StackPanel>
</UserControl>  

      

        
        
        
      

    

Public Class TestUserControl

    Public Property TestProp As Color
        Get
            Return GetValue(TestPropProperty)
        End Get

        Set(ByVal value As Color)
            SetValue(TestPropProperty, value)
        End Set
    End Property

    Public Shared ReadOnly TestPropProperty As DependencyProperty = DependencyProperty.Register("TestProp",
                                                                                                GetType(Color),
                                                                                                GetType(TestUserControl),
                                                                                                New PropertyMetadata(Color.FromRgb(&H0, &HBC, &HC4)))

End Class

      

        
        
        
      

    

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'ElementName=MyUserControl'. BindingExpression:Path=TestProp; DataItem=null; target element is 'SolidColorBrush' (HashCode=29677729); target property is 'Color' (type 'Color')