.net XML Serialization - Storing Reference instead of Object Copy

Question

• In .Net/C# Application, I have data structures which have references to each other.
• When I serialize them, .Net Serializes all references with separate object copies.
• In Following Example, I am trying to serialize to Array of 'Person'

• A 'Person' may have reference to another person.

  public class Person
  {
      public string Name;
      public Person Friend;
  }
  
  Person p1 = new Person();
  p1.Name = "John";
  
  Person p2 = new Person();
  p2.Name = "Mike";
  
  p1.Friend = p2;
  
  Person[] group = new Person[] { p1, p2 };
  XmlSerializer ser = new XmlSerializer(typeof(Person[]));
  using (TextWriter tw = new StreamWriter("test.xml"))
      ser.Serialize(tw,group );
  
  //above code generates following xml
  
  <ArrayOfPerson>
    <Person>
      <Name>John</Name>
      <Friend>
        <Name>Mike</Name>
      </Friend>
    </Person>
    <Person>
      <Name>Mike</Name>
    </Person>
  </ArrayOfPerson>
  

• In above code, the same 'Mike' object are there on two places, since there are two references for the same object.

• While deserializing, they become two different objects, which is not exact state when they are serialized.
• I want to avoid this and have only copy of object in serialized xml, and all references should refer to this copy. After deserialization , i want to get back, same old data structure.
• Is it Possible ?

Answer 1

It is not possible with XmlSerializer. You could achieve this with DataContractSerializer using the PreserveObjectReferences property. You may take a look at this post which explains the details.

Here's a sample code:

public class Person
{
    public string Name;
    public Person Friend;
}

class Program
{
    static void Main(string[] args)
    {
        Person p1 = new Person();
        p1.Name = "John";

        Person p2 = new Person();
        p2.Name = "Mike";
        p1.Friend = p2;
        Person[] group = new Person[] { p1, p2 };

        var serializer = new DataContractSerializer(group.GetType(), null, 
            0x7FFF /*maxItemsInObjectGraph*/, 
            false /*ignoreExtensionDataObject*/, 
            true /*preserveObjectReferences : this is where the magic happens */, 
            null /*dataContractSurrogate*/);
        serializer.WriteObject(Console.OpenStandardOutput(), group);
    }
}

This produces the following XML:

<ArrayOfPerson z:Id="1" z:Size="2" xmlns="http://schemas.datacontract.org/2004/07/ToDelete" xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns:z="http://schemas.microsoft.com/2003/10/Serialization/">
    <Person z:Id="2">
        <Friend z:Id="3">
            <Friend i:nil="true"/>
            <Name z:Id="4">Mike</Name>
        </Friend>
        <Name z:Id="5">John</Name>
    </Person>
    <Person z:Ref="3" i:nil="true"/>
</ArrayOfPerson>

Now set PreserveObjectReferences to false in the constructor and you will get this:

<ArrayOfPerson xmlns="http://schemas.datacontract.org/2004/07/ToDelete" xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
    <Person>
        <Friend>
            <Friend i:nil="true"/>
            <Name>Mike</Name>
        </Friend>
        <Name>John</Name>
    </Person>
    <Person>
        <Friend i:nil="true"/>
        <Name>Mike</Name>
    </Person>
</ArrayOfPerson>

It is worth mentioning that the XML produced this way is not interoperable and can only be deserialized with a DataContractSerializer (same remark as with the BinaryFormatter).