Silverlight DataGrid有关问题

Silverlight DataGrid问题！
Silverlight 里怎样获取DataGrid中 RowDetailsTemplate 中的控件值？
比如：

C# code

<sdk:DataGrid.RowDetailsTemplate>   <DataTemplate>      <StackPanel x:Name="stackReason" Height="50" Orientation="Horizontal" HorizontalAlignment="Left" VerticalAlignment="Center">       <TextBlock x:Name="tbR1" Text="展示信息:" VerticalAlignment="Center"></TextBlock>       <TextBlock x:Name="UpId" Visibility="Collapsed" DataContext="{Binding ReasonID}"></TextBlock>       <TextBox x:Name="txtReasonContent" Text="{Binding ReasonContent}" Width="300" Height="50"></TextBox>       <Button x:Name="btnUpdate" Content="保存" Width="50" Click="btnUpdate_Click"></Button>      </StackPanel>    </DataTemplate></sdk:DataGrid.RowDetailsTemplate>