[WP8.1] - ItemTemplateSelector - Not Binding the Fullmode

Question

I am developing Windows phone 8.1 RT  app.   I  am using Combobox.  There  are more than 20 Operators to be bind to the Combbox.  when we tab  on the Combobox, App should open the all 20 opetaros in full mode.  I need  two  different  templates, should use one template (image and textblock) when  items opens  in Full mode and other template (only TextBlock) when a item selected  among  full mode items. DataTemplateSelector is inherited and created new DataTemplateSelector.  ItemTemplateSelector is assigned with newly inherited DataTemplateSelector.  Below the is code used.

<ComboBox Grid.Row="3" Grid.Column="0" Margin="15 5 0 0" 
                  ItemsSource="{Binding Operators}"  SelectedItem="{Binding SelectedOperator, Mode=TwoWay}"
                  Style="{StaticResource FullModeComboBoxStyle1}"   ItemContainerStyle="{StaticResource FullModeComboBoxItemStyle1}"
                  VerticalAlignment="Top"
                  Height="65"
                  ItemTemplateSelector="{StaticResource ExploreTemplateSelector}"
                  />

public class ExploreTemplateSelector : DataTemplateSelector
    {
        public DataTemplate DropdownItemsTemplate { get; set; }
        public DataTemplate SelectedItemTemplate { get; set; }

        protected override DataTemplate SelectTemplateCore(object item, DependencyObject container)
        {
            var parent = container;

            while (parent != null && !(parent is ComboBoxItem) && !(parent is ComboBox))
                parent = VisualTreeHelper.GetParent(parent);

            var inDropDown = (parent is ComboBoxItem);

            return inDropDown
                ? DropdownItemsTemplate
                : SelectedItemTemplate;

        }
}

       <DataTemplate x:Key="OperatorDataTemplate">
            <StackPanel Orientation="Horizontal" Margin="5 5 0 0" Height="Auto">
                <Image Source="{Binding ImageUri}" Height="35" Width="60" VerticalAlignment="Top" />
                <TextBlock Text="{Binding Name}" Style="{StaticResource ComboboxTextBlockStyle}" Margin="5 0 0 0"   Width="120" VerticalAlignment="Top" TextWrapping="Wrap"/>
            </StackPanel>
        </DataTemplate>

        <DataTemplate x:Key="SelectedOperatorDataTemplate">
            <TextBlock Text="{Binding Name}" Style="{StaticResource ComboboxTextBlockStyle}" Margin="5 0 0 0"   Width="120" VerticalAlignment="Top" TextWrapping="Wrap"/>
        </DataTemplate>

        <class:ExploreTemplateSelector x:Key="ExploreTemplateSelector" DropdownItemsTemplate="{StaticResource SelectedOperatorDataTemplate}"
                                       SelectedItemTemplate="{StaticResource SelectedOperatorDataTemplate}"
                                       />

Items  are not binding when we tab  the Combobox,  showing list of namespace.  But selecting a item in full mode, SelectTemplateCore is hitted and Selected item is showed using SelectedItemTemplate.  But SelectTemplateCore  is not hitted when binding Operators.

What is the problem in this code? Why DropDownItemsTemplate is not used to bind the items?

Thanks in advance