How to sum a field in the data table

 private void BindGrid()

    {

        // Set your connection String here

        SqlConnection cn = new SqlConnection("Data Source=.;Initial Catalog=Demo_eLogsVA4;Integrated Security=True");

        //Write a select Query

        cn.Open();

        string q = "Select Convert(nVarchar,A.Rec_No) CRec,A.Acc_Name Descr,A.Quantity Qty,Convert(nVarchar,A.Sell_Rate) SRate,Sell_CName Sname , Sell_CCode SCCode, Sell_CValue SCValue ,Convert(nVarchar,A.Quantity*A.Sell_Rate*Sell_CValue) SAmount, A.Service_Tax STax,Convert(nVarchar,(A.Quantity*A.Sell_Rate*Sell_CValue)* A.Service_Tax/100)Tax_Amount,Convert(nVarchar,((A.Quantity*A.Sell_Rate*Sell_CValue)+(A.Quantity*A.Sell_Rate*Sell_CValue)* A.Service_Tax/100)) Net From eLogs_Invoice_Charges_Details A Where A.Invoice_Code='INV991'";

        DataSet dset = new DataSet();

        SqlDataAdapter da = new SqlDataAdapter(q, cn);        

        DataTable dtable = new DataTable();

        da.Fill(dtable);

        //Bind grid.

        GridView1.DataSource = dtable;

        object obj = dtable.Compute("SUM(Net)", string.Empty);

        double value = (double)obj;

        GridView1.DataBind();

        GridView1.FooterRow.Cells[10].Text = value.ToString();

        cn.Close();

    }