How to sum two values in sql server2008

Hi frnds,

    I have to select a column value from a table and with that i have to sum another value using c#.net


   i m selecting like this

     string dbConn = ConfigurationManager.ConnectionStrings["CMC"].ConnectionString;
        SqlConnection sqlConn = new SqlConnection(dbConn);
        sqlConn.Open();

  DateTime ExDate = DateTime.ParseExact(txtExpiryDate.Text, "dd-MM-yyyy", CultureInfo.InvariantCulture);
   string strSQL2 = "select [Opening_Qty] from [Stock] where [ItemName]='" + txtItemName.Text + "' AND [BatchNo]='" + txtBatchNo.Text + "' AND         [ExpiryDate]='" + ExDate + "'";
             SqlCommand cmdUpdate = new SqlCommand(strSQL2, sqlConn);
             SqlDataReader drOpeningBalance = cmdUpdate.ExecuteReader();
             if (drOpeningBalance.Read())
            {
          string    OpeningQty = drOpeningBalance[0].ToString();

            }
  


and


  string strSQL1 = " Update [Stock] set [Opening_Qty]='" +( OpeningQty +(Convert.ToInt32(txtQty.Text)))+ "' where [ItemName]='" + txtItemName.Text + "' and [BatchNo]='" + txtBatchNo.Text + "'";
              SqlCommand commandReader = new SqlCommand(strSQL1, sqlConn);
              commandReader.ExecuteNonQuery();



like above lines i m summing up two values ...But its not working...Any suggestion pls...