XElement root = XElement.Load(Server.MapPath("~/1.xml"));
var query = from f in root.Elements("CerType")
              select new { TypeID = f.Element("TypeID").Value, TypeName = f.Element("TypeName").Value };
DropDownList1.DataValueField = "TypeID";
DropDownList1.DataTextField = "TypeName";
DropDownList1.DataSource = query.ToList();
DropDownList1.DataBind();

------解决方案--------------------
C# code

            XDocument doc = XDocument.Load("c:\\1.txt");
            var vv = from r in doc.Descendants("CerType") select r;
            var vs = from r in doc.Descendants("CerType") select new { id = r.Element("TypeID").Value, name = r.Element("TypeName").Value };
            comboBox1.DisplayMember = "id";//或者是"name"
            comboBox1.DataSource = vs.ToArray();