<MenuItem Header="Add" Click="AddNewSnapshotButton_Click" 
Visibility="{Binding ElementName=MainTreeView, Path=SelectedItem, Converter={StaticResource ShowHideMenuItem}}">
                        <MenuItem.Icon>
                            <Image Source="/Icon;component/Resources/55.png"  Width="16"></Image>
                        </MenuItem.Icon>

                    </MenuItem>


[ValueConversion(typeof(object), typeof(Visibility))]
   public class ShowHideMenuItemConverter : IValueConverter
   {
      public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
      {
         return (value.GetType() == typeof(MYEntity) ? Visibility.Visible : Visibility.Collapsed);

      }

      public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
      {
         throw new NotSupportedException();
      }
   }