XmlChoiceIdentifierAttribute Class

Specifies that the member can be further detected by using an enumeration.

Syntax

[System.AttributeUsage(System.AttributeTargets.Property | System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.ReturnValue | System.AttributeTargets.All)]
public class XmlChoiceIdentifierAttribute : Attribute

Remarks

The XML schema element definition named xsi:choice is used to define a complex element that can contain only one child in an instance (maxoccurs = 1). That child can be one of several types, and it can have one of several names. Each name is associated with a specific type; however, several names can be associated with the same type. Because of this, an instance of such an element is indistinct. For example, consider the following schema fragment that defines such an indistinct element named MyChoice.

Example

 <xsd:complexType name="MyChoice">
  <xsd:sequence>
  <xsd:choice minOccurs="0" maxOccurs="1">
  <xsd:element minOccurs="1" maxOccurs="1" name="ChoiceOne" type="xsd:string" />
  <xsd:element minOccurs="1" maxOccurs="1" name="ChoiceTwo" type="xsd:string" />
  </xsd:choice>
  </xsd:sequence>
 </xsd:complexType>

The System.Xml.Serialization.XmlChoiceIdentifierAttribute allows you to assign a special enumeration value to each instance of the member. You must either create the enumeration yourself or it can be generated by the XML Schema Definition Tool (Xsd.exe). The following C# code shows how the System.Xml.Serialization.XmlChoiceIdentifierAttribute is applied to an Item field; the XmlChoiceIdentifierAttribute.MemberName property identifies the field that contains the enumeration that is further used to detect the choice.

Example

 public class Choices{
  [XmlChoiceIdentifier("ItemType")]
  [XmlChoiceIdentifier("ChoiceOne")]
  [XmlChoiceIdentifier("ChoiceTwo")]
  public string MyChoice;

  // Do not serialize this next field:
  [XmlIgnore]
  public ItemChoiceType ItemType;
 }
 // Do not include this enumeration in the XML schema.
 [XmlType(IncludeInSchema = false)]
 public enum ItemChoiceType{
  ChoiceOne,
  ChoiceTwo,
 }

When this code is in place, you can serialize and deserialize this class by setting the ItemType field to an appropriate enumeration. For example, to serialize the Choice class, the C# code resembles the following.

Example

 Choices mc = new Choices();
 mc.MyChoice = "Item Choice One";
 mc.ItemType = ItemChoiceType.ChoiceOne;

When deserializing, the C# code resembles the following:

Example

 MyChoice mc = (MyChoice) myXmlSerializer.Deserialize(myReader);
 if(mc.ItemType == ItemChoiceType.ChoiceOne)
  {
      // Handle choice one.
  }
 if(mc.ItemType == ItemChoiceType.ChoiceTwo)
  {
      // Handle choice two.
  }
 if(mc.ItemType != null)
  {
      throw CreateUnknownTypeException(mc.Item);
  }

There is a second scenario when the System.Xml.Serialization.XmlChoiceIdentifierAttribute is used. In the following schema, the member is a field that returns an array of items (maxOccurs="unbounded"). The array can contain objects of the first choice ("D-a-t-a"), and of the second choice ("MoreData").

Example

 <xsd:complexType name="MyChoice">
  <xsd:sequence>
  <xsd:choice minOccurs="0" maxOccurs="unbounded">
  <xsd:element minOccurs="1" maxOccurs="1" name="D-a-t-a" type="xsd:string" />
  <xsd:element minOccurs="1" maxOccurs="1" name="MoreData" type="xsd:string" />
  </xsd:choice>
  </xsd:sequence>
 </xsd:complexType>

The resulting class then uses a field to return an array of items. For each item in the array, a corresponding ItemChoiceType enumeration must also be found. The matching enumerations are contained in the array returned by the ItemsElementName field.

Example

 public class MyChoice {
  [System.Xml.Serialization.XmlElementAttribute("D-a-t-a", typeof(string), IsNullable=false)]
  [System.Xml.Serialization.XmlElementAttribute("MoreData", typeof(string), IsNullable=false)]
  [System.Xml.Serialization.XmlChoiceIdentifierAttribute("ItemsElementName")]
  public string[] Items;
  [System.Xml.Serialization.XmlElementAttribute(IsNullable=false)]
  [System.Xml.Serialization.XmlIgnoreAttribute()]
  public ItemsChoiceType[] ItemsElementName;
 }
 [System.Xml.Serialization.XmlTypeAttribute(IncludeInSchema=false)]
 public enum ItemsChoiceType {
  [System.Xml.Serialization.XmlEnumAttribute("D-a-t-a")]
  Data,
  MoreData,
 }

When deserializing an object that includes a range of choices, use a control structure (such as an if...then...else structure) to determine how to deserialize a particular value. In the control structure, check the enumeration value and deserialize the value accordingly.

Requirements

Namespace: System.Xml.Serialization
Assembly: System.Xml (in System.Xml.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0