Binding with ElementName does not work

Question

Here is the XAML:

<DataGrid Grid.Column="0"  AutoGenerateColumns="False" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" ItemsSource="{Binding Path=Columns}"
              x:Name="ColumnsGrid" RowHeaderWidth="0">
        <DataGrid.Columns>
            <DataGridTextColumn Width="*" Binding="{Binding Path=Header}" 
                                Header="{Binding ElementName=ColumnsGrid, Path=DataContext.Count, StringFormat=Columns ({0}), diag:PresentationTraceSources.TraceLevel=High}"/>
        </DataGrid.Columns>  
    </DataGrid>

Binding returns error: System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element.

What do I miss?

Update:

Here is the answer: http://blogs.msdn.com/b/jaimer/archive/2008/11/22/forwarding-the-datagrid-s-datacontext-to-its-columns.aspx

What is happening here? The Columns collection is just a property in the Datagrid; this collection is not in the logical (or visual) tree, therefore the DataContext is not being inherited, which leads to there being nothing to bind to.

Update 2: Good article about DataGrid's caveats: http://blogs.msdn.com/b/vinsibal/archive/2009/04/07/5-random-gotchas-with-the-wpf-datagrid.aspx

Answer 1

Binding on DataGridColumn for Header abd Visibility properties needs special treatment...

See this...

Bind datagrid column visibility MVVM

Answer 2

If you are interested in the Count property of the object that is referred to in the DataContext you try and use regular databinding:

<DataGrid Grid.Column="0"  AutoGenerateColumns="False" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" ItemsSource="{Binding Path=Columns}"
          x:Name="ColumnsGrid" RowHeaderWidth="0">
    <DataGrid.Columns>
        <DataGridTextColumn Width="*" Binding="{Binding Path=Header}" 
                            Header="{Binding Path=Count, StringFormat=Columns ({0}), diag:PresentationTraceSources.TraceLevel=High}"/>
    </DataGrid.Columns>  
</DataGrid>

EDIT

Apparently the columns of a datagrid are not part of the Visual Tree So using ElementName and RelativeSource will not work. Perhaps you should add the property to the object the column is bound to.