System.Xml.Serialization.XmlChoiceIdentifierAttribute Class

Summary

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

C# Syntax:

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

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 ambiguous. For example, consider the following schema fragment that defines such an ambiguous element named MyChoices.

          <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 the conceptual topic at MSDN: xmlschemadefinitiontoolxsdexe. The C# code below shows how the XmlChoiceIdentifierAttribute is applied to an Item field; the XmlChoiceIdentifierAttribute.MemberName property identifies the field that contains the enumeration that is further used to disambiguate the choice.

          public class Choices{
           [XmlChoiceIdentifier("itemType")
           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{
           None,
           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 MyChoice class, the C# code resembles the following.

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

When deserializing, the C# code might resemble 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 schema below, 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.

The following example serializes a class named Choices that includes two fields, MyChoice and ManyChoices . The XmlChoiceIdentifierAttribute is applied to each field specifying (through the XmlChoiceIdentifierAttribute.MemberName property) another class member that gets or sets an enumeration that disambiguates 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;
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());
      	}
      
   }
}

    

System.Xml.Serialization Namespace

System.Xml.Serialization.XmlChoiceIdentifierAttribute Member List:

Public Constructors

ctor #1	Overloaded: .ctor() Default constructor. This constructor is called by derived class constructors to initialize state in this type. Initializes a new instance of the XmlChoiceIdentifierAttribute class.
ctor #2	Overloaded: .ctor(string name) Initializes a new instance of the XmlChoiceIdentifierAttribute class.

Public Properties

MemberName	Read-write Gets or sets the name of the field that returns the enumeration to use when disambiguating types.
TypeId (inherited from System.Attribute)	Read-only See base class member description: System.Attribute.TypeId When implemented in a derived class, gets a unique identifier for this Attribute.

Public Methods

Equals (inherited from System.Object)	See base class member description: System.Object.Equals Derived from System.Object, the primary base class for all objects.
GetHashCode (inherited from System.Attribute)	See base class member description: System.Attribute.GetHashCode Returns the hash code for this instance.
GetType (inherited from System.Object)	See base class member description: System.Object.GetType Derived from System.Object, the primary base class for all objects.
IsDefaultAttribute (inherited from System.Attribute)	See base class member description: System.Attribute.IsDefaultAttribute When overridden in a derived class, returns an indication whether the value of this instance is the default value for the derived class.
Match (inherited from System.Attribute)	See base class member description: System.Attribute.Match When overridden in a derived class, returns a value indicating whether this instance equals a specified object.
ToString (inherited from System.Object)	See base class member description: System.Object.ToString Derived from System.Object, the primary base class for all objects.

Protected Methods

Finalize (inherited from System.Object)	See base class member description: System.Object.Finalize Derived from System.Object, the primary base class for all objects.
MemberwiseClone (inherited from System.Object)	See base class member description: System.Object.MemberwiseClone Derived from System.Object, the primary base class for all objects.

Hierarchy:

• System.Object
• System.Attribute
• System.Xml.Serialization.XmlChoiceIdentifierAttribute

System.Xml.Serialization.XmlChoiceIdentifierAttribute Member Details

Summary

Initializes a new instance of the XmlChoiceIdentifierAttribute class.

Default constructor. This constructor is called by derived class constructors to initialize state in this type.

C# Syntax:

public XmlChoiceIdentifierAttribute();

The following example serializes a class named Choices that includes two fields, MyChoice and ManyChoices . The XmlChoiceIdentifierAttribute is applied to each field specifying (through the XmlChoiceIdentifierAttribute.MemberName property) another class member that gets or sets an enumeration that disambiguates 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;
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());
      	}
      
   }
}

    

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Summary

Initializes a new instance of the XmlChoiceIdentifierAttribute class.

C# Syntax:

public XmlChoiceIdentifierAttribute(    string name );

name

The member name that returns the enumeration used to disambiguate a choice.

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Summary

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

C# Syntax:

public string MemberName {get; set;}

At least one member must be present in the enumeration returned by the field named in the XmlChoiceIdentifierAttribute.MemberName value. By default, that enumeration name takes the name of the field that the XmlChoiceIdentifierAttribute is applied to.

The following example serializes a class named Choices that includes two fields, MyChoice and ManyChoices . The XmlChoiceIdentifierAttribute is applied to each field specifying (through the XmlChoiceIdentifierAttribute.MemberName property) another class member that gets or sets an enumeration that disambiguates 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;
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());
      	}
      
   }
}

    

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Inherited
See base class member description: System.Attribute.TypeId

Summary

When implemented in a derived class, gets a unique identifier for this Attribute.

C# Syntax:

public virtual object TypeId {get;}

As implemented, this identifier is merely the Type of the attribute. However, it is intended that the unique identifier be used to identify two attributes of the same type.

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Inherited
See base class member description: System.Object.Equals

C# Syntax:

public virtual bool Equals(    object obj );

For more information on members inherited from System.Object click on the link above.

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Inherited
See base class member description: System.Object.Finalize

C# Syntax:

~XmlChoiceIdentifierAttribute();

For more information on members inherited from System.Object click on the link above.

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Inherited
See base class member description: System.Attribute.GetHashCode

Summary

Returns the hash code for this instance.

C# Syntax:

public override int GetHashCode();

Return Value:

A 32-bit signed integer hash code.

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Inherited
See base class member description: System.Object.GetType

C# Syntax:

public Type GetType();

For more information on members inherited from System.Object click on the link above.

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Inherited
See base class member description: System.Attribute.IsDefaultAttribute

Summary

When overridden in a derived class, returns an indication whether the value of this instance is the default value for the derived class.

C# Syntax:

public virtual bool IsDefaultAttribute();

Return Value:

true if this instance is the default attribute for the class; otherwise, false.

The default implementation of this class returns false, and must be implemented in the derived class to be useful to that class.

The implementation of this method in a derived class compares the value of this instance to a standard, default value obtained by some means, then returns a Boolean value that indicates whether the value of this instance is equal to the standard. The standard value is typically coded as a constant in the implementation, or stored programmatically in a field used by the implementation.

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Inherited
See base class member description: System.Attribute.Match

Summary

When overridden in a derived class, returns a value indicating whether this instance equals a specified object.

C# Syntax:

public virtual bool Match(    object obj );

obj

An Object to compare with this instance of Attribute.

Return Value:

true if this instance equals obj; otherwise, false.

This method determines if one Attribute equals another. Its default implementation is the same as Attribute.Equals, which performs a value and reference comparison. Override this method to implement support for attribute values, such as flags or bitfields, that consist of components that are meaningful in themselves. For example, consider an attribute whose value is a binary field divided into a bitfield of flags. Two instances of this attribute have one flag in set in common while all the other flags differ. The Equal method cannot determine that the two instances have the same flag set, but the Match method can.

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Inherited
See base class member description: System.Object.MemberwiseClone

C# Syntax:

protected object MemberwiseClone();

For more information on members inherited from System.Object click on the link above.

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Inherited
See base class member description: System.Object.ToString

C# Syntax:

public virtual string ToString();

For more information on members inherited from System.Object click on the link above.

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