XmlChoiceIdentifierAttribute Class

Definition

Namespace:

System.Xml.Serialization

Assembly:

System.Xml.XmlSerializer.dll

Source:

XmlChoiceIdentifierAttribute.cs

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

public ref class XmlChoiceIdentifierAttribute : Attribute

[System.AttributeUsage(System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.Property | System.AttributeTargets.ReturnValue, AllowMultiple=false)]
public class XmlChoiceIdentifierAttribute : Attribute

[<System.AttributeUsage(System.AttributeTargets.Field | System.AttributeTargets.Parameter | System.AttributeTargets.Property | System.AttributeTargets.ReturnValue, AllowMultiple=false)>]
type XmlChoiceIdentifierAttribute = class
    inherit Attribute

Public Class XmlChoiceIdentifierAttribute
Inherits Attribute

Inheritance

Object

Attribute

XmlChoiceIdentifierAttribute

Attributes

AttributeUsageAttribute

Examples

The following example serializes a class named Choices that includes two fields, MyChoice and ManyChoices. The XmlChoiceIdentifierAttribute is applied to each field that specifies (through the MemberName property) another class member that gets or sets an enumeration that detects the member value. The MyChoice field can be set to a single value, with a corresponding enumeration member found in the EnumType field. The ManyChoices field returns an array of objects. The ChoiceArray field returns an array of enumeration values. For each array member in the ManyChoices field, a corresponding member is found in the array returned by the ChoiceArray field.

#using <System.dll>
#using <System.xml.dll>

using namespace System;
using namespace System::Xml;
using namespace System::Xml::Serialization;
using namespace System::IO;

[XmlType(IncludeInSchema=false)]

public enum class ItemChoiceType
{
   None, Word, Number, DecimalNumber
};

public enum class MoreChoices
{
   None, Item, Amount, Temp
};

public ref class Choices
{
public:

   // The MyChoice field can be set to any one of 
   // the types below. 

   [XmlChoiceIdentifier("EnumType")]
   [XmlElement("Word",String::typeid)]
   [XmlElement("Number",Int32::typeid)]
   [XmlElement("DecimalNumber",Double::typeid)]
   Object^ MyChoice;

   // Don't serialize this field. The EnumType field
   // contains the enumeration value that corresponds
   // to the MyChoice field value.

   [XmlIgnore]
   ItemChoiceType EnumType;

   // The ManyChoices field can contain an array
   // of choices. Each choice must be matched to
   // an array item in the ChoiceArray field.

   [XmlChoiceIdentifier("ChoiceArray")]
   [XmlElement("Item",String::typeid)]
   [XmlElement("Amount",Int32::typeid)]
   [XmlElement("Temp",Double::typeid)]
   array<Object^>^ManyChoices;

   // TheChoiceArray field contains the enumeration
   // values, one for each item in the ManyChoices array.

   [XmlIgnore]
   array<MoreChoices>^ChoiceArray;
};

void SerializeObject( String^ filename );
void DeserializeObject( String^ filename );
int main()
{
   SerializeObject( "Choices.xml" );
   DeserializeObject( "Choices.xml" );
}

void SerializeObject( String^ filename )
{
   XmlSerializer^ mySerializer = gcnew XmlSerializer( Choices::typeid );
   TextWriter^ writer = gcnew StreamWriter( filename );
   Choices^ myChoices = gcnew Choices;

   // Set the MyChoice field to a string. Set the
   // EnumType to Word.
   myChoices->MyChoice = "Book";
   myChoices->EnumType = ItemChoiceType::Word;

   // Populate an object array with three items, one
   // of each enumeration type. Set the array to the 
   // ManyChoices field.
   array<Object^>^strChoices = {"Food",5,98.6};
   myChoices->ManyChoices = strChoices;

   // For each item in the ManyChoices array, add an
   // enumeration value.
   array<MoreChoices>^ itmChoices = {MoreChoices::Item,MoreChoices::Amount,MoreChoices::Temp};
   myChoices->ChoiceArray = itmChoices;
   mySerializer->Serialize( writer, myChoices );
   writer->Close();
}

void DeserializeObject( String^ filename )
{
   XmlSerializer^ ser = gcnew XmlSerializer( Choices::typeid );

   // A FileStream is needed to read the XML document.
   FileStream^ fs = gcnew FileStream( filename,FileMode::Open );
   Choices^ myChoices = safe_cast<Choices^>(ser->Deserialize( fs ));
   fs->Close();

   // Disambiguate the MyChoice value using the enumeration.
   if ( myChoices->EnumType == ItemChoiceType::Word )
   {
      Console::WriteLine( "Word: {0}", myChoices->MyChoice->ToString() );
   }
   else
   if ( myChoices->EnumType == ItemChoiceType::Number )
   {
      Console::WriteLine( "Number: {0}", myChoices->MyChoice->ToString() );
   }
   else
   if ( myChoices->EnumType == ItemChoiceType::DecimalNumber )
   {
      Console::WriteLine( "DecimalNumber: {0}", myChoices->MyChoice->ToString() );
   }

   // Disambiguate the ManyChoices values using the enumerations.
   for ( int i = 0; i < myChoices->ManyChoices->Length; i++ )
   {
      if ( myChoices->ChoiceArray[ i ] == MoreChoices::Item )
            Console::WriteLine( "Item: {0}", myChoices->ManyChoices[ i ] );
      else
      if ( myChoices->ChoiceArray[ i ] == MoreChoices::Amount )
            Console::WriteLine( "Amount: ", myChoices->ManyChoices[ i ]->ToString() );
      if ( myChoices->ChoiceArray[ i ] == MoreChoices::Temp )
            Console::WriteLine( "Temp: {0}", myChoices->ManyChoices[ i ]->ToString() );
   }
}

using System;
using System.Xml;
using System.Xml.Serialization;
using System.IO;

public class Choices{
   // The MyChoice field can be set to any one of 
   // the types below. 
   [XmlChoiceIdentifier("EnumType")]
   [XmlElement("Word", typeof(string))]
   [XmlElement("Number", typeof(int))]
   [XmlElement("DecimalNumber", typeof(double))]
   public object MyChoice;

   // Don't serialize this field. The EnumType field
   // contains the enumeration value that corresponds
   // to the MyChoice field value.
   [XmlIgnore]
   public ItemChoiceType EnumType;

   // The ManyChoices field can contain an array
   // of choices. Each choice must be matched to
   // an array item in the ChoiceArray field.
   [XmlChoiceIdentifier("ChoiceArray")]
   [XmlElement("Item", typeof(string))]
   [XmlElement("Amount", typeof(int))]
   [XmlElement("Temp", typeof(double))]
   public object[] ManyChoices;

   // TheChoiceArray field contains the enumeration
   // values, one for each item in the ManyChoices array.
   [XmlIgnore]
   public MoreChoices[] ChoiceArray;
}

[XmlType(IncludeInSchema=false)]
public enum ItemChoiceType{
   None,
   Word, 
   Number,
   DecimalNumber
}

public enum MoreChoices{
   None,
   Item,
   Amount,
   Temp
}

public class Test{
   static void Main(){
      Test t = new Test();
      t.SerializeObject("Choices.xml");
      t.DeserializeObject("Choices.xml");
   }

   private void SerializeObject(string filename){
      XmlSerializer mySerializer = 
      new XmlSerializer(typeof(Choices));
      TextWriter writer = new StreamWriter(filename);
      Choices myChoices = new Choices();

      // Set the MyChoice field to a string. Set the
      // EnumType to Word.
      myChoices.MyChoice= "Book";
      myChoices.EnumType = ItemChoiceType.Word;

      // Populate an object array with three items, one
      // of each enumeration type. Set the array to the 
      // ManyChoices field.
      object[] strChoices = new object[]{"Food",  5, 98.6};
      myChoices.ManyChoices=strChoices;

      // For each item in the ManyChoices array, add an
      // enumeration value.
      MoreChoices[] itmChoices = new MoreChoices[]
      {MoreChoices.Item, 
      MoreChoices.Amount,
      MoreChoices.Temp};
      myChoices.ChoiceArray=itmChoices;
      
      mySerializer.Serialize(writer, myChoices);
      writer.Close();
   }

   private void DeserializeObject(string filename){
      XmlSerializer ser = new XmlSerializer(typeof(Choices));

      // A FileStream is needed to read the XML document.
      FileStream fs = new FileStream(filename, FileMode.Open);
      Choices myChoices = (Choices)
      ser.Deserialize(fs);
      fs.Close();

      // Disambiguate the MyChoice value using the enumeration.
      if(myChoices.EnumType == ItemChoiceType.Word){
           Console.WriteLine("Word: " +  
            myChoices.MyChoice.ToString());
        }
      else if(myChoices.EnumType == ItemChoiceType.Number){
           Console.WriteLine("Number: " +
            myChoices.MyChoice.ToString());
        }
      else if(myChoices.EnumType == ItemChoiceType.DecimalNumber){
           Console.WriteLine("DecimalNumber: " +
            myChoices.MyChoice.ToString());
        }

      // Disambiguate the ManyChoices values using the enumerations.
      for(int i = 0; i<myChoices.ManyChoices.Length; i++){
      if(myChoices.ChoiceArray[i] == MoreChoices.Item)
        Console.WriteLine("Item: " + (string) myChoices.ManyChoices[i]);
      else if(myChoices.ChoiceArray[i] == MoreChoices.Amount)
        Console.WriteLine("Amount: " + myChoices.ManyChoices[i].ToString());
      if(myChoices.ChoiceArray[i] == MoreChoices.Temp)
        Console.WriteLine("Temp: " + (string) myChoices.ManyChoices[i].ToString());
        }
   }
}

Imports System.Xml
Imports System.Xml.Serialization
Imports System.IO

Public Class Choices
   ' The MyChoice field can be set to any one of 
   ' the types below. 
   <XmlChoiceIdentifier("EnumType"), _
   XmlElement("Word", GetType(String)), _
   XmlElement("Number", GetType(Integer)), _
   XmlElement("DecimalNumber", GetType(double))> _
   Public MyChoice As Object 

   ' Don't serialize this field. The EnumType field
   ' contains the enumeration value that corresponds
   ' to the MyChoice field value.
   <XmlIgnore> _
   Public EnumType As ItemChoiceType 

   'The ManyChoices field can contain an array
   ' of choices. Each choice must be matched to
   ' an array item in the ChoiceArray field.
   <XmlChoiceIdentifier("ChoiceArray"), _
   XmlElement("Item", GetType(string)), _
   XmlElement("Amount", GetType(Integer)), _
   XmlElement("Temp", GetType(double))> _
   Public ManyChoices() As Object

   ' TheChoiceArray field contains the enumeration
   ' values, one for each item in the ManyChoices array.
   <XmlIgnore> _
   Public ChoiceArray() As MoreChoices
End Class

<XmlType(IncludeInSchema:=false)> _
Public Enum ItemChoiceType
   None
   Word 
   Number
   DecimalNumber
End Enum

<XmlType(IncludeInSchema:=false)> _
Public Enum MoreChoices
   None
   Item
   Amount
   Temp
End Enum

Public Class Test
   Shared Sub Main()
      Dim t  As Test = New Test()
      t.SerializeObject("Choices.xml")
      t.DeserializeObject("Choices.xml")
   End Sub

   private Sub SerializeObject(filename As string)
      Dim mySerializer As XmlSerializer = _
      New XmlSerializer(GetType(Choices))
      Dim writer As TextWriter = New StreamWriter(filename)
      Dim myChoices As Choices = New Choices()

      ' Set the MyChoice field to a string. Set the
      ' EnumType to Word.
      myChoices.MyChoice= "Book"
      myChoices.EnumType = ItemChoiceType.Word

      ' Populate an object array with three items, one
      ' of each enumeration type. Set the array to the 
      ' ManyChoices field.
      Dim strChoices () As Object = New object(){"Food",  5, 98.6}
      myChoices.ManyChoices=strChoices

      ' For each item in the ManyChoices array, add an
      ' enumeration value.
      Dim itmChoices () As MoreChoices = New MoreChoices() _
      {MoreChoices.Item, _
      MoreChoices.Amount, _
      MoreChoices.Temp}
      myChoices.ChoiceArray=itmChoices
      
      mySerializer.Serialize(writer, myChoices)
      writer.Close()
   End Sub

   private Sub DeserializeObject(filename As string)
      Dim ser As XmlSerializer = New XmlSerializer(GetType(Choices))
      

      ' A FileStream is needed to read the XML document.
      Dim fs As FileStream = New FileStream(filename, FileMode.Open)
      
      Dim myChoices As Choices = CType(ser.Deserialize(fs), Choices)

      fs.Close()

      ' Disambiguate the MyChoice value Imports the enumeration.
      if myChoices.EnumType = ItemChoiceType.Word Then
           Console.WriteLine("Word: " & _
           myChoices.MyChoice.ToString())
        
      else if myChoices.EnumType = ItemChoiceType.Number Then 
           Console.WriteLine("Number: " & _
            myChoices.MyChoice.ToString())
        
      else if myChoices.EnumType = ItemChoiceType.DecimalNumber Then
         Console.WriteLine("DecimalNumber: " & _
            myChoices.MyChoice.ToString())
        End If

      ' Disambiguate the ManyChoices values Imports the enumerations.
      Dim i As Integer
      for i = 0 to myChoices.ManyChoices.Length -1
      if myChoices.ChoiceArray(i) = MoreChoices.Item Then
        Console.WriteLine("Item: " +  _
        myChoices.ManyChoices(i).ToString())
      else if myChoices.ChoiceArray(i) = MoreChoices.Amount Then
        Console.WriteLine("Amount: " + _
        myChoices.ManyChoices(i).ToString())
      else if (myChoices.ChoiceArray(i) = MoreChoices.Temp)
        Console.WriteLine("Temp: " + _
        myChoices.ManyChoices(i).ToString())
        End If
        Next i
      
   End Sub
End Class

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.

<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 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 XmlChoiceIdentifierAttribute is applied to an Item field; the MemberName 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 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").

<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.

Constructors

XmlChoiceIdentifierAttribute()	Initializes a new instance of the XmlChoiceIdentifierAttribute class.
XmlChoiceIdentifierAttribute(String)	Initializes a new instance of the XmlChoiceIdentifierAttribute class.

Properties

MemberName	Gets or sets the name of the field that returns the enumeration to use when detecting types.
TypeId	When implemented in a derived class, gets a unique identifier for this Attribute. (Inherited from Attribute)

Methods

Equals(Object)	Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute)
GetHashCode()	Returns the hash code for this instance. (Inherited from Attribute)
GetType()	Gets the Type of the current instance. (Inherited from Object)
IsDefaultAttribute()	When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute)
Match(Object)	When overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute)
MemberwiseClone()	Creates a shallow copy of the current Object. (Inherited from Object)
ToString()	Returns a string that represents the current object. (Inherited from Object)

See also

• XmlAttributes
• Introducing XML Serialization
• How to: Specify an Alternate Element Name for an XML Stream
• Controlling XML Serialization Using Attributes
• Examples of XML Serialization
• XML Schema Definition Tool (Xsd.exe)