DatatemplateSelector，Listbox stretch

Question

我在ListBox中使用DatatemplateSelector。

我的數據不會strecth根據需要（灰色圓圈必須在列表框的右邊）

<ListBox HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch" SelectionChanged="ListBoxMessagesSelectionChanged" x:Name="listmy" > 
    <ListBox.ItemTemplate> 
     <DataTemplate> 
     <local:SelectorForSearchFriend Content="{Binding}"> 
      <local:SelectorForSearchFriend.User> 
      <DataTemplate> 
       <Grid HorizontalAlignment="Stretch"> 
       <Grid.ColumnDefinitions> 
        <ColumnDefinition Width="3*"/> 
        <ColumnDefinition Width="15*"/> 
        <ColumnDefinition Width="Auto"/> 
       </Grid.ColumnDefinitions> 
       <Image Grid.Column="0" Width="100" Height="70"/> 
       <TextBlock Name="Title" Text="{Binding Title}"/> 
       <Ellipse HorizontalAlignment="Right" Fill="#FFB8B8B8" Height="20" Width="20" StrokeThickness="0" Visibility="{Binding IsOnline}" Grid.Column="2" Margin="0,-20,0,0"/> 
       </Grid> 
      </DataTemplate> 
      </local:SelectorForSearchFriend.User> 
     </DataTemplate> 
    </ListBox.ItemTemplate> 
    <ListBox.ItemContainerStyle> 
     <Style TargetType="ListBoxItem"> 
     <Setter Property="HorizontalAlignment" Value="Stretch"/> 
     <Setter Property="HorizontalContentAlignment" Value="Stretch"/> 
     <Setter Property="Margin" Value="0,4,0,4"/> 
     </Style> 
    </ListBox.ItemContainerStyle> 
    </ListBox> 

Answer 1

<phone:PhoneApplicationPage.Resources> 
    <Style TargetType="Grid"> 
     <Setter Property="HorizontalAlignment" Value="Stretch"/> 
    </Style> 
    <Style TargetType="local:SelectorForSearchFriend"> 
     <Setter Property="Template"> 
      <Setter.Value> 
       <ControlTemplate TargetType="local:SelectorForSearchFriend"> 
        <ContentPresenter ContentTemplate="{TemplateBinding ContentTemplate}" Content="{TemplateBinding Content}" Margin="{TemplateBinding Padding}" VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/> 
       </ControlTemplate> 
      </Setter.Value> 
     </Setter> 
     <Setter Property="HorizontalAlignment" Value="Stretch"/> 
     <Setter Property="VerticalAlignment" Value="Stretch"/> 
    </Style> 
    </phone:PhoneApplicationPage.Resources>