System.Xml [00 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00] 1.0.5000.0 2.0.0.0 4.0.0.0 To be added System.Attribute System.AttributeUsage(System.AttributeTargets.Property | System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.ReturnValue | System.AttributeTargets.All)

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.

`<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 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 is applied to an Item field; the property identifies the field that contains the enumeration that is further used to detect the choice.

`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.

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

When deserializing, the C# code resembles the following:

`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 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").

`<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.

`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.

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

- Constructor
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  Initializes a new instance of the class.

- Constructor
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  Initializes a new instance of the class.

  The member name that returns the enumeration used to detect a choice.
- Property
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  At least one member must be present in the enumeration returned by the field named in the value. By default, that enumeration name takes the name of the field that the is applied to.

  Gets or sets the name of the field that returns the enumeration to use when detecting types.