focus in textbox after text asp.net c# -

I am trying to focus in text box after text in asp.net c # but I am unable to do this

My C # code is given below, please tell me where I am wrong: -

  text box txtindex = (TextBox) RptTask.Items [Nextindex] .FindControl ("TxtDuration"); Txtindex.focus (); Apply Focus on Text in    
 
 
  Page_PreRender  
   // Pre-render code is given below, protected Zero Page_PreRender (Object Sender, EventArgs E) {TryBox txtindex = (TextBox) RptTask.Items [Nextindex]. FindControl ("TxtDuration"); Txtindex.focus (); } Hold (exception EE) {}}  

Hope this will help!