Deserialize XML with multiple types

Question

I'm trying to deserialize XML where some same name tags have different xsi types:

<user-defined-data-row>
  <field name="entity">
    <field-value xsi:type="field-text-valueType">
      <value>Test</value>
    </field-value>
  </field>
  <field name="expiry_date">
    <field-value xsi:type="field-date-valueType">
      <value>2001-10-07</value>
    </field-value>
  </field>
</user-defined-data-row>

This is easily achieved by deserializing the xml into this model:

[XmlRoot(ElementName = "field-value", Namespace = "http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0")]
[XmlType("field-text-valueType")]
public class Fieldvalue
{
    [XmlElement(ElementName = "value", Namespace = "http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0")]
    public string Value { get; set; }
}

The only thing that differs are the types in the XML:

field-text-valueType

field-date-valueType

How can I make the C# class interpret both types using something like

[XmlType("field-text-valueType")]

EDIT: deserializing not serializing

Answer 1

The xsi:type attributes you are seeing in your XML are standard W3C XML Schema attributes that allow an element to explicitly specify its type; for details see here. As explained in Xsi:type Attribute Binding Support, XmlSerializer supports this mechanism for deserialization of polymorphic types, specifically by use of XmlIncludeAttribute.

First, define an abstract base class FieldValue as follows:

public static class XmlNamespaces
{
    public const string Crsoftwareinc = "http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0";
}

[XmlRoot("field-value", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("field-value", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlInclude(typeof(TextFieldValue)), 
XmlInclude(typeof(DateFieldValue))]
public abstract partial class FieldValue
{
    // It's not necessary to have this in the base class but I usually find it convenient.
    public abstract object GetValue();
}

Next, for each possible xsi:type="XXX" value, define a derived type of FieldValue whose XmlTypeAttribute.TypeName matches the xsi:type value. Decorate the base class with [XmlInclude(typeof(TDerivedFieldValue))] attributes for each (already shown above):

[XmlRoot("field-text-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("field-text-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
public class TextFieldValue : FieldValue
{
    [XmlElement("value")]
    public string Value { get; set; }

    public override object GetValue() { return Value; }
}

[XmlRoot("field-date-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("field-date-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
public class DateFieldValue : FieldValue
{
    [XmlElement("value", DataType = "date")]
    public DateTime Value { get; set; }

    public override object GetValue() { return Value; }
}

Then define the containing type corresponding to <field> and other, higher elements as follows:

[XmlRoot("field", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("field", Namespace = XmlNamespaces.Crsoftwareinc)]
public class Field
{
    [XmlAttribute("name")]
    public string Name { get; set; }

    [XmlElement("field-value")]
    public FieldValue FieldValue { get; set; }
}

[XmlRoot("user-defined-data-row", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("user-defined-data-row", Namespace = XmlNamespaces.Crsoftwareinc)]
public class UserDefinedDataRow
{
    [XmlElement("field")]
    public List<Field> Fields { get; set; }
}

// The XML for the root object is not shown so this is just a stub
[XmlRoot("root", Namespace = XmlNamespaces.Crsoftwareinc)]
[XmlType("root", Namespace = XmlNamespaces.Crsoftwareinc)]
public class RootObject
{
    [XmlElement("user-defined-data-row")]
    public List<UserDefinedDataRow> Rows { get; set; }
}

Notes:

• If the base class FieldValue has a namespace specified in via XmlTypeAttribute.Namespace, then the derived classes must also, or else an error will get thrown by XmlSerializer.

  Once an [XmlType] namespace is defined, it automatically applies to all serialized properties, so it isn't necessary to specify the same namespace via [XmlElement(Namespace = "http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0")] attributes.

• I got tired of repeatedly typing the namespace "http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0" and so I extracted it into a constant.

• Other derived types of FieldType can be added easily, e.g.:

  [XmlRoot("field-decimal-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
  [XmlType("field-decimal-valueType", Namespace = XmlNamespaces.Crsoftwareinc)]
  public class DecimalFieldValue : FieldValue
  {
      [XmlElement("value")]
      public decimal Value { get; set; }
  
      public override object GetValue() { return Value; }
  }
  
  [XmlInclude(typeof(DecimalFieldValue))]
  public abstract partial class FieldValue { }
  

  Don't forget to add [XmlInclude(typeof(DecimalFieldValue))] when doing so.

• If you have been given an XSD for the XML you are trying to deserialize that specifies the possible types of <field-value> via e.g. an <xsd:extension> element as shown in Generating XML Documents from XML Schemas: Abstract Types, then xsd.exe will generate classes that include an appropriate type hierarchy. But if you only have the XML, then xsd.exe and Paste XML as Classes will not generate a correct type hierarchy using whatever xsi:type attributes are present.

  For more about this limitation see xsi:type attribute messing up C# XML deserialization.

• Your XML is not well-formed because it omits a declaration for the xsi: namespace. Also, a default namespace xmlns="http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0" is not defined so none of the elements are actually in this namespace. Thus I assume your XML is a fragment of some larger document that is valid, e.g.

  <root 
   xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
   xmlns="http://www.crsoftwareinc.com/xml/ns/titanium/common/v1_0">
    <user-defined-data-row>
        <!-- Remainder as shown in the question -->
    </user-defined-data-row>
  </root>
  

Sample working .Net fiddle here.