getting listbox.selectedindex by clicking on checkbox

this is my user template in listbox. i want to have "listbox.selectedindex" on clicking any checkbox of listbox. i want to knw of which row,checkbox is selected.like on click event of checkbox,it should focus the whole selected row.

     <UserControl.Resources>
      <DataTemplate x:Key="datatemp">
       <StackPanel Orientation="Horizontal" Width="200" >
        <TextBlock Text="{Binding VmName}" Width="129" Visibility="Visible" />
        <CheckBox  Name="cb"  IsThreeState="False" Checked="off_chek_select"  IsChecked="{Binding IsCheck, Mode=TwoWay}"  Margin="6,0,18,6" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"/>  
      </StackPanel>
   </DataTemplate>
</UserControl.Resources>