error in using BtnSubmit.Attributes("onclick") in asp.net
<p>can anybody tell me what is problem in</p>

<p>BtnSubmit.Attributes("onclick") = string.Format("document.getElementById(\'{0}\').value= document.getElementById(\'{1}\').value;", this.HiddenField1.ClientID, this.FileUpload1.ClientID);</p>

<p>it is giving me error 'Non invocable memberusing System.Web.UI.WebControls.Attributes cannot be used like method.'</p>
