Webová služba - choice element

Question

Zdravím,

jak přiřadit hodnotu pro choice element - viz. příklad:

ws_test.AType my_ws = new AType();

my_ws.AData = new ADataType();
my_ws.AData.Item ??

namespace ws_test
{
    [Serializable]
    [DebuggerStepThrough]
    [XmlType(AnonymousType = true, Namespace = "urn:cz:aaa:bbb:schemas:zzz:v1")]
    [GeneratedCode("System.Xml", "4.0.30319.1")]
    [DesignerCategory("code")]
    public class ATypeReq
    {
        public ATypeReq();

        public ADataType AData { get; set; }
    }

    [Serializable]
    [DesignerCategory("code")]
    [GeneratedCode("System.Xml", "4.0.30319.1")]
    [DebuggerStepThrough]
    [XmlType(Namespace = "urn:cz:aaa:bbb:schemas:xxx:v2")]
    public class ADataType
    {
        public ADataType();

        [XmlElement("AElement", typeof(ADataTypeAA))]
        [XmlElement("BElement", typeof(ADataTypeBB))]
        public AStavAbstractType Item { get; set; }
    }
}

  <xs:complexType name="ADataType">
    <xs:annotation>
      <xs:documentation xmlns:xml="http://www.w3.org/XML/1998/namespace" xml:lang="cs">               
            </xs:documentation>
    </xs:annotation>
    <xs:choice>
      <xs:element name="AElement">
        <xs:complexType>
          <xs:complexContent mixed="false">
            <xs:extension base="edi:AStavAbstractType">
              <xs:sequence>
                <xs:element minOccurs="0" name="AA" type="edi:AAStavType" />
              </xs:sequence>
            </xs:extension>
          </xs:complexContent>
        </xs:complexType>
      </xs:element>
      <xs:element name="BElement">
        <xs:complexType>
          <xs:complexContent mixed="false">
            <xs:extension base="edi:AStavAbstractType">
              <xs:sequence>
                <xs:element name="BB" type="edi:BBStavType" />
              </xs:sequence>
            </xs:extension>
          </xs:complexContent>
        </xs:complexType>
      </xs:element>
    </xs:choice>
  </xs:complexType>

Děkuji.