How to access child control of template in code behind ?

Hi All,

            I am using button template in resource dictionary and this template  key  access  in button i want to child control access in code behind (.cs) such as:

            Template in  Resource Dictionary :

             <ControlTemplate x:Key="btnTemplate" TargetType="Button">
                   <Grid>
                            <TextBlock x:Name="tb" Height="25" Width="150" ></TextBlock>
                   </Grid>
              </ControlTemplate>

 

             .xaml  

             <Button x:Name="btn" Template="{DynamicResource  btnTemplate}" Height="25" Width="150" >

                    I want to   textblock (x:Name="tb") name access in code behind.

    Thanks

            

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aniruddha