How to Convert Grid View Rows as XML String

public string ConvertCircleXML()
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("");
            sb.Append("<Circle>");
            foreach (GridViewRow Item in GrdCircle.Rows)
            {
                CheckBox GrdChkBox = (CheckBox)(Item.FindControl("ChkCircle"));
                if (GrdChkBox.Checked == true)
                {
                    sb.Append("<test>");
                    string CircleId = GrdCircle.DataKeys[Item.RowIndex].Values[0].ToString();
                    string CorpBillingId = GrdCircle.DataKeys[Item.RowIndex].Values[1].ToString();
                    TextBox TxtGrdBillingName = (TextBox)(Item.FindControl("TxtBillingName1"));
                    TextBox TxtGrdBillingAmount = (TextBox)(Item.FindControl("TxtBillingAmount1"));
                    sb.Append("<CircleId>" + CircleId + "</CircleId><CorpBillingId>" + CorpBillingId + "</CorpBillingId><TxtGrdBillingName>" + TxtGrdBillingName.Text + "</TxtGrdBillingName><TxtGrdBillingAmount>" + TxtGrdBillingAmount.Text + " </TxtGrdBillingAmount><UserId>" + Session["UserId"].ToString() + "</UserId>");
                    sb.Append("</test>");
                }
            }
            sb.Append("</Circle>");
            return sb.ToString();
        }