How Do I Add Datacolumn In Existing Datatable And Populate It?

public virtual DataTable cart(ArrayList dt)
        {
            int loop = 0;
            DataTable result = new DataTable();
            SqlCommand cmd;
            DataTable dtrow = new DataTable();
            while (loop < dt.Count)
            {
                cmd = new SqlCommand("getcart @prname");
                cmd.Parameters.AddWithValue("@prname", dt[loop].ToString());
                dtrow = data.getdata(ref cmd);
                result.Merge(dtrow);
                loop++;
            }
            DataColumn qty = new DataColumn ("qty");
            result.Columns.Add(qty);
            DataRow dr;
            foreach (DataRow row in result.Rows)
            {
                dr = result.NewRow();
                dr["qty"] = 1;
                result.Rows.Add(dr);
            }

           
            
            return result;
        }