Question

Trying to serialize a union-like data-type. There is an enum field indicating the type of data stored in the union, and a variety of possible field types.

The desired result is DataContractSerializer produced XML which contains just the enum, and the relevant field.

Possible solutions, none of which have been attempted yet, are:

• Use a custom serializer and mark the union properties with a custom attribute, similar to this question. The custom serializer would strip out the members not required.
• Use ISerializationSurrogate and serialize a different object which just contains the relevant data.
• Don’t use separate fields in the union, use one object field (this could be used as part of the implementation of the ISerializationSurrogate approach).
• Other… ?

For example:

[DataContract]
public class WCFTestUnion
{
    public enum EUnionType
    {
        [EnumMember]
        Bool,
        [EnumMember]
        String,
        [EnumMember]
        Dictionary,
        [EnumMember]
        Invalid
    };

    EUnionType unionType = EUnionType.Invalid;

    bool boolValue = true;
    string stringValue = "Hello";
    IDictionary<object, object> dictionaryValue = null;

    // Could use custom attribute here ?
    [DataMember]
    public bool BoolValue
    {
        get { return this.boolValue; }
        set { this.boolValue = value; }
    }

    // Could use custom attribute here ?
    [DataMember]
    public string StringValue
    {
        get { return this.stringValue; }
        set { this.stringValue = value; }
    }

    // Could use custom attribute here ?
    [DataMember]
    public IDictionary<object, object> DictionaryValue
    {
        get { return this.dictionaryValue; }
        set { this.dictionaryValue = value; }
    }

    [DataMember]
    public EUnionType UnionType
    {
        get { return this.unionType; }
        set { this.unionType = value; }
    }
} // Ends class WCFTestUnion

Test

    class TestSerializeUnion
    {
        internal static void Test()
        {
            Console.WriteLine("===TestSerializeUnion.Test()===");

            WCFTestUnion u = new WCFTestUnion();
            u.UnionType = WCFTestUnion.EUnionType.Dictionary;
            u.DictionaryValue = new Dictionary<object, object>();
            u.DictionaryValue[1] = "one";
            u.DictionaryValue["two"] = 2;

            System.Runtime.Serialization.DataContractSerializer serialize = new System.Runtime.Serialization.DataContractSerializer(typeof(WCFTestUnion));
            System.IO.Stream stream = new System.IO.MemoryStream();

            serialize.WriteObject(stream, u);

            stream.Seek(0, System.IO.SeekOrigin.Begin);
            byte[] buffer = new byte[stream.Length];
            int length = checked((int)stream.Length);
            int read = stream.Read(buffer, 0, length);
            while (read < stream.Length)
            {
                read += stream.Read(buffer, 0, length - read);
            }

            string xml = Encoding.Default.GetString(buffer);

            System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
            doc.LoadXml(xml);

            System.Xml.XmlTextWriter xmlwriter = new System.Xml.XmlTextWriter(Console.Out);
            xmlwriter.Formatting = System.Xml.Formatting.Indented;

            doc.WriteContentTo(xmlwriter);
            xmlwriter.Flush();

            Console.WriteLine();
        }
    } // Ends class TestSerializeUnion

Output:

<WCFTestUnion xmlns="http://schemas.datacontract.org/2004/07/WCFTestServiceContracts" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <BoolValue>true</BoolValue>
  <DictionaryValue xmlns:a="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
    <a:KeyValueOfanyTypeanyType>
      <a:Key i:type="b:int" xmlns:b="http://www.w3.org/2001/XMLSchema">1</a:Key>
      <a:Value i:type="b:string" xmlns:b="http://www.w3.org/2001/XMLSchema">one</a:Value>
    </a:KeyValueOfanyTypeanyType>
    <a:KeyValueOfanyTypeanyType>
      <a:Key i:type="b:string" xmlns:b="http://www.w3.org/2001/XMLSchema">two</a:Key>
      <a:Value i:type="b:int" xmlns:b="http://www.w3.org/2001/XMLSchema">2</a:Value>
    </a:KeyValueOfanyTypeanyType>
  </DictionaryValue>
  <StringValue>Hello </StringValue>
  <UnionType>Dictionary</UnionType>
</WCFTestUnion>

Desired Output (only field being used is serialized, along with enum):

<WCFTestUnion xmlns="http://schemas.datacontract.org/2004/07/WCFTestServiceContracts" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <DictionaryValue xmlns:a="http://schemas.microsoft.com/2003/10/Serialization/Arrays">
    <a:KeyValueOfanyTypeanyType>
      <a:Key i:type="b:int" xmlns:b="http://www.w3.org/2001/XMLSchema">1</a:Key>
      <a:Value i:type="b:string" xmlns:b="http://www.w3.org/2001/XMLSchema">one</a:Value>
    </a:KeyValueOfanyTypeanyType>
    <a:KeyValueOfanyTypeanyType>
      <a:Key i:type="b:string" xmlns:b="http://www.w3.org/2001/XMLSchema">two</a:Key>
      <a:Value i:type="b:int" xmlns:b="http://www.w3.org/2001/XMLSchema">2</a:Value>
    </a:KeyValueOfanyTypeanyType>
  </DictionaryValue>
  <UnionType>Dictionary</UnionType>
</WCFTestUnion>

Answer 1

You do have several options here. What you use depends on the complexity of this scenario (where else you have to do something like this, how often and in what ways you have to serialize this data, performance, etc.) Take a look at these options, ask away if you have more questions, but mostly, I recommend you just play and experiment with multiple strategies from the list below before picking one or a hybrid solution.

• Use a data contract resolver. Provides a mechanism for dynamically mapping types to and from wire representations during serialization and deserialization, giving you flexibility to support far more types than you can out-of-the-box.

• Use IObjectReference. You can have a class which implements and returns a reference to a different object after it has been deserialized.

• Use a data contract surrogate. This is different from the serialization surrogates you’re referring to, but also similar. I think these might work out nicely for you