Disable / Enable context menu item depending on the list item

I have UserControl

c ListView

. Also, I have a listViewItems.cs class with DisplayMemberBinding for GridView

in ListView

.

Each ListView item has a context menu. Now I am trying to enable / disable context menu items if the value in the ListViewItems class isnull

I have tried binding a property IsEnabled

to a boolean ShowResItemEn

in the ListViewItems.cs class but it doesn't work.

DataOutput.xaml

<ListView.Resources>
    <ContextMenu x:Name="cmListView" x:Key="ItemContextMenu" DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}}"> 
        <MenuItem x:Name="itmRes"
                    Header="Reservierungen anzeigen"
                    IsEnabled="{Binding PlacementTarget.SelectedItem.ShowResItemEn, RelativeSource={RelativeSource FindAncestor,AncestorType=ContextMenu}}"
                    Command="{Binding ShowResItemCmd}"
                    CommandParameter="{Binding RelativeSource={RelativeSource AncestorType=ContextMenu}, Path=PlacementTarget.SelectedItem}" >
        </MenuItem>
    </ContextMenu>
</ListView.Resources>

      

        
        
        
      

    

ListViewItems.cs class

public Boolean ShowResItemEn
{
   get
   {
       return (auftrNr[0] == null) ? false : true; 
   }
}