DataGrid row Ascending and Descending sorting

try
{
Viewstate["strFieldName"] = e.SortExpression ;
if( Viewstate["strSort"] == null ||
Convert.ToString( Viewstate["strSort"] ) != "DESC" )
{
Session["strSort"] = "DESC";
}
else
{
Session["strSort"] = "ASC";
}

dvTest = new DataView( dsTest.Tables[0] , "", "",
DataViewRowState.CurrentRows);
dvTest.Sort = Session["strFieldName"] + " " + Session["strSort"] ;
dgList.DataSource = dvTest ;
dgList.DataBind();
}
catch (Exception objExp)
{
// Showing the Error
lblError.Text = objExp.Message.ToString();
}