convert sql datasource to data table and compute on data table.

        Dim dvSql As DataView = DirectCast(SqlDataSourceCustomerInvoice.Select(DataSourceSelectArguments.Empty), DataView)
        Dim dt As New DataTable
        dt = dvSql.ToTable


        Dim amt As Double = dt.Compute("Sum(RentAmt)", "")
        Literal1.Text = "Total Amount: " & amt.ToString()