Dynamic binding of MenuItem icon in XAML. Is it possible?

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XAML

WPF

Binding

Trying to set up the binding for a menu item's icon so that I can vary it according to the number (and state) of items selected.

I've tried a number of variations for the Image Source binding including the three shown below as well as a number of others I can't be bothered to repeat here.

Nothing seems to work. Binding is not established, all I get are variations on a theme of, "Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Controls.ListView', AncestorLevel='1''. BindingExpression:Path=SelectedItems; DataItem=null; target element is 'Image' (Name='startIcon'); target property is 'Source' (type 'ImageSource')" with varying error codes (mainly 4).

It would seem I'm missing something obvious, but I can't see what it is.

<ListView  x:Name="grdTasks"
           ItemContainerStyle="{StaticResource SimpleListBoxItem}"
           DockPanel.Dock="Top"
           HorizontalAlignment="Stretch"
           ScrollViewer.HorizontalScrollBarVisibility="Disabled"
           ScrollViewer.VerticalScrollBarVisibility="Auto"
           HorizontalContentAlignment="Stretch"
           VerticalContentAlignment="Stretch" Margin="2">

  <ListView.ContextMenu>
    <ContextMenu  x:Name="ItemsMenu"
      DataContext="{Binding Path=PlacementTarget.DataContext,
                    RelativeSource={RelativeSource Self}}" >
      <MenuItem x:Name="start"
                Header="Start"
                Command="{Binding Path=StartCommand}"
                CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu},
                                           Path=PlacementTarget.SelectedItems}">

        <MenuItem.Icon>
          <Image x:Name="startIcon" Height="16" Width="16"
                 Source="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu, AncestorLevel=2},
                                                  Path=PlacementTarget.SelectedItems,
                                                  Converter={StaticResource GroupImageConverter}}">
          </Image>
        </MenuItem.Icon>


      </MenuItem>
    </ContextMenu>
  </ListView.ContextMenu>

</ListView>

...as well as targetting a named element.

<MenuItem.Icon>
  <Image x:Name="startIcon" Height="16" Width="16"
         Source="{Binding ElementName=grdTasks,
                          Path=SelectedItems,
                          Converter={StaticResource GroupImageConverter}}">
  </Image>
</MenuItem.Icon>

...and this. Although I'd half expect this not to work because (according to our favourite search tool), "context menu is not part of the visual tree".

<MenuItem.Icon>
  <Image x:Name="startIcon" Height="16" Width="16"
         Source="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListView}},
                                          Path=SelectedItems,
                                          Converter={StaticResource GroupImageConverter}}">
  </Image>
</MenuItem.Icon>