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XmlChoiceIdentifierAttribute Class

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

For a list of all members of this type, see XmlChoiceIdentifierAttribute Members.

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

[Visual Basic]
<AttributeUsage(AttributeTargets.Property Or AttributeTargets.Field _
   Or AttributeTargets.Parameter Or AttributeTargets.ReturnValue)>
Public Class XmlChoiceIdentifierAttribute
   Inherits Attribute
[C#]
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field
   | AttributeTargets.Parameter | AttributeTargets.ReturnValue)]
public class XmlChoiceIdentifierAttribute : Attribute
[C++]
[AttributeUsage(AttributeTargets::Property |
   AttributeTargets::Field | AttributeTargets::Parameter |
   AttributeTargets::ReturnValue)]
public __gc class XmlChoiceIdentifierAttribute : public Attribute
[JScript]
public
   AttributeUsage(AttributeTargets.Property | AttributeTargets.Field |
   AttributeTargets.Parameter | AttributeTargets.ReturnValue)
class XmlChoiceIdentifierAttribute extends Attribute

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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 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 XML Schema Definition Tool (Xsd.exe). The C# code below 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 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.

Example

[Visual Basic, C#, C++] 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 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.

[Visual Basic] 
Imports System
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

[C#] 
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());
          }
      
   }
}

[C++] 
#using <mscorlib.dll>
#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 __value enum ItemChoiceType
{
   None,
   Word, 
   Number,
   DecimalNumber
};

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

public __gc class Choices
{
public:
   // The MyChoice field can be set to any one of 
   // the types below. 
   [XmlChoiceIdentifier("EnumType")]
   [XmlElement("Word", __typeof(String))]
   [XmlElement("Number", __typeof(Int32))]
   [XmlElement("DecimalNumber", __typeof(Double))]
   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", __typeof(String))]
   [XmlElement("Amount", __typeof(Int32))]
   [XmlElement("Temp", __typeof(Double))]
   Object*    ManyChoices[];

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

void SerializeObject(String* filename);
void DeserializeObject(String* filename);

int main(){
   SerializeObject(S"Choices.xml");
   DeserializeObject(S"Choices.xml");
}

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= S"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[] = {S"Food", __box(5), __box(98.6) };
   myChoices->ManyChoices = strChoices;

   // For each item in the ManyChoices array, add an
   // enumeration value.
   MoreChoices    itmChoices[] =
   {MoreChoices::Item, 
   MoreChoices::Amount,
   MoreChoices::Temp};
   myChoices->ChoiceArray = itmChoices;

   mySerializer->Serialize(writer, myChoices);
   writer->Close();
}

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 = __try_cast<Choices*>( ser->Deserialize(fs));
   fs->Close();

   // Disambiguate the MyChoice value using the enumeration.
   if(myChoices->EnumType == ItemChoiceType::Word)
   {
      Console::WriteLine( S"Word: {0}", myChoices->MyChoice->ToString());
   }
   else if( myChoices->EnumType == ItemChoiceType::Number)
   {
      Console::WriteLine( S"Number: {0}", myChoices->MyChoice->ToString());
   }
   else if( myChoices->EnumType == ItemChoiceType::DecimalNumber)
   {
      Console::WriteLine( S"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( S"Item: {0}", myChoices->ManyChoices[i]);    
      else if ( myChoices->ChoiceArray[i] == MoreChoices::Amount )
         Console::WriteLine( S"Amount: ", myChoices->ManyChoices[i]->ToString());
      if(myChoices->ChoiceArray[i] == MoreChoices::Temp)
         Console::WriteLine(S"Temp: {0}" , myChoices->ManyChoices[i]->ToString());
   }

}

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Namespace: System.Xml.Serialization

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework

Assembly: System.Xml (in System.Xml.dll)

See Also

XmlChoiceIdentifierAttribute Members | System.Xml.Serialization Namespace

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