SQL Server长时间查询引起的ASP.NET网页timeout报错

using (SqlConnection conn = new SqlConnection(connString))
         {
             SqlDataAdapter sda = new SqlDataAdapter(SqlString, conn);
             sda.SelectCommand.CommandTimeout = 250; //延长timeout时间
             DataSet ds = new DataSet();
             sda.Fill(ds);
             return ds;
         }