.NET Serialization Ordering

Question

I am trying to serialize some objects using XmlSerializer and inheritance but I am having some problems with ordering the outcome.

Below is an example similar to wha

Answer 1

Like Nader said, maybe think about making a more loose-coupled design. However, in my case, loose-coupling was not appropriate. Here's my class hierarchy, and how I propose to solve the problem without using custom serialization or DTOs.

In my project, I'm constructing a whole bunch of objects to represent pieces of an XML document that will be submitted via a web service. There are a very large number of pieces. Not all are sent with every request (actually, in this example, I'm modeling a response, but the concepts are the same). These pieces are used much like building blocks to assemble a request (or disassemble a response, in this case). So here's an example of using aggregation/encapsulation to accomplish the desired ordering despite the inheritance hierarchy.

[Serializable]
public abstract class ElementBase
{
    // This constructor sets up the default namespace for all of my objects. Every
    // Xml Element class will inherit from this class.
    internal ElementBase()
    {
        this._namespaces = new XmlSerializerNamespaces(new XmlQualifiedName[] {
            new XmlQualifiedName(string.Empty, "urn:my-default-namespace:XSD:1")
        });
    }

    [XmlNamespacesDeclaration]
    public XmlSerializerNamespaces Namespaces { get { return this._namespaces; } }
    private XmlSerializationNamespaces _namespaces;
}


[Serializable]
public abstract class ServiceBase : ElementBase
{
    private ServiceBase() { }

    public ServiceBase(Guid requestId, Guid? asyncRequestId = null, Identifier name = null)
    {
        this._requestId = requestId;
        this._asyncRequestId = asyncRequestId;
        this._name = name;
    }

    public Guid RequestId
    {
        get { return this._requestId;  }
        set { this._requestId = value;  }
    }
    private Guid _requestId;

    public Guid? AsyncRequestId
    {
        get { return this._asyncRequestId; }
        set { this._asyncRequestId = value; }
    }
    private Guid? _asyncRequestId;

    public bool AsyncRequestIdSpecified
    {
        get { return this._asyncRequestId == null && this._asyncRequestId.HasValue; }
        set { /* XmlSerializer requires both a getter and a setter.*/ ; }
    }

    public Identifier Name
    {
        get { return this._name; }
        set { this._name; }
    }
    private Identifier _name;
}


[Serializable]
public abstract class ServiceResponseBase : ServiceBase
{
    private ServiceBase _serviceBase;

    private ServiceResponseBase() { }

    public ServiceResponseBase(Guid requestId, Guid? asyncRequestId = null, Identifier name = null, Status status = null)
    {
        this._serviceBase = new ServiceBase(requestId, asyncRequestId, name);
        this._status = status;
    }

    public Guid RequestId
    {
        get { return this._serviceBase.RequestId; }
        set { this._serviceBase.RequestId = value; }
    }

    public Guid? AsyncRequestId
    {
        get { return this._serviceBase.AsyncRequestId; }
        set { this._serviceBase.AsyncRequestId = value; }
    }

    public bool AsynceRequestIdSpecified
    {
        get { return this._serviceBase.AsyncRequestIdSpecified; }
        set { ;  }
    }

    public Identifier Name
    {
        get { return this._serviceBase.Name; }
        set { this._serviceBase.Name = value; }
    }

    public Status Status
    {
        get { return this._status; }
        set { this._status = value; }
    }
}

[Serializable]
[XmlRoot(Namespace = "urn:my-default-namespace:XSD:1")]
public class BankServiceResponse : ServiceResponseBase
{
    // Determines if the class is being deserialized.
    private bool _isDeserializing;

    private ServiceResponseBase _serviceResponseBase;

    // Constructor used by XmlSerializer.
    // This is special because I require a non-null List<T> of items later on.
    private BankServiceResponse()
    { 
        this._isDeserializing = true;
        this._serviceResponseBase = new ServiceResponseBase();
    }

    // Constructor used for unit testing
    internal BankServiceResponse(bool isDeserializing = false)
    {
        this._isDeserializing = isDeserializing;
        this._serviceResponseBase = new ServiceResponseBase();
    }

    public BankServiceResponse(Guid requestId, List<BankResponse> responses, Guid? asyncRequestId = null, Identifier name = null, Status status = null)
    {
        if (responses == null || responses.Count == 0)
            throw new ArgumentNullException("The list cannot be null or empty", "responses");

        this._serviceResponseBase = new ServiceResponseBase(requestId, asyncRequestId, name, status);
        this._responses = responses;
    }

    [XmlElement(Order = 1)]
    public Status Status
    {
        get { return this._serviceResponseBase.Status; }
        set { this._serviceResponseBase.Status = value; }
    }

    [XmlElement(Order = 2)]
    public Guid RequestId
    {
        get { return this._serviceResponseBase.RequestId; }
        set { this._serviceResponseBase.RequestId = value; }
    }

    [XmlElement(Order = 3)]
    public Guid? AsyncRequestId
    {
        get { return this._serviceResponseBase.AsyncRequestId; }
        set { this._serviceResponseBase.AsyncRequestId = value; }
    }

    [XmlIgnore]
    public bool AsyncRequestIdSpecified
    {
        get { return this._serviceResponseBase.AsyncRequestIdSpecified; }
        set { ; } // Must have this for XmlSerializer.
    }

    [XmlElement(Order = 4)]
    public Identifer Name
    {
         get { return this._serviceResponseBase.Name; }
         set { this._serviceResponseBase.Name; }
    }

    [XmlElement(Order = 5)]
    public List<BankResponse> Responses
    {
        get { return this._responses; }
        set
        {
            if (this._isDeserializing && this._responses != null && this._responses.Count > 0)
                this._isDeserializing = false;

            if (!this._isDeserializing && (value == null || value.Count == 0))
                throw new ArgumentNullException("List cannot be null or empty.", "value");

            this._responses = value;
        }
    }
    private List<BankResponse> _responses;
}

So, while I have to create properties for all of the contained classes, I can delegate any custom logic I might have within the contained class(es) property setters/getters by simply using the contained class's properties when the leaf class's properties are accessed. Since there's no inheritance, I can decorate all the properties of the leaf class with the XmlElementAttribute attribute and use any ordering that I see fit.

---

UPDATE:

I came back to revisit this article because my design decisions about using class inheritance came back to bite me again. While my solution above does work, I'm using it, I really think that Nader's solution is the best and should be considered before the solution I presented. In fact, I'm +1'ing him today! I really like his answer, and if I ever have the opportunity to refactor my current project, I will definitely be separating the business object from the serialization logic for objects that would otherwise benefit greatly from inheritance in order to simplify the code and make it easier for others to use and understand.

Thanks for posting your response Nader, as I think many will find it very instructive and useful.

Answer 2

This post is quite old now, but I had a similar problem in WCF recently, and found a solution similar to Steve Cooper's above, but one that does work, and presumably will work for XML Serialization too.

If you remove the XmlElement attributes from the base class, and add a copy of each property with a different name to the derived classes that access the base value via the get/set, the copies can be serialized with the appropriate name assigned using an XmlElementAttribute, and will hopefully then serialize in the default order:

public class SerializableBase
{
   public bool Property1 { get; set;}
   public bool Property3 { get; set;}
}

[XmlRoot("Object")]
public class SerializableObject : SerializableBase
{
  [XmlElement("Property1")]
  public bool copyOfProperty1 
  { 
    get { return base.Property1; }
    set { base.Property1 = value; }
  }

  [XmlElement]
  public bool Property2 { get; set;}

  [XmlElement("Property3")]
  public bool copyOfProperty3
  { 
    get { return base.Property3; }
    set { base.Property3 = value; }
  }
}

I also added an Interface to add to the derived classes, so that the copies could be made mandatory:

interface ISerializableObjectEnsureProperties
{
  bool copyOfProperty1  { get; set; }
  bool copyOfProperty2  { get; set; }
}

This is not essential but means that I can check everything is implemented at compile time, rather than checking the resultant XML. I had originally made these abstract properties of SerializableBase, but these then serialize first (with the base class), which I now realise is logical.

This is called in the usual way by changing one line above:

public class SerializableObject : SerializableBase, ISerializableObjectEnsureProperties

I've only tested this in WCF, and have ported the concept to XML Serialization without compiling, so if this doesn't work, apologies, but I would expect it to behave in the same way - I'm sure someone will let me know if not...

Answer 3

It looks like the XmlSerializer class serializes the base type and then derived types in that order and is only respecting the Order property within each class individually. Even though the order is not totally what you want, it should still Deserialize properly. If you really must have the order just like that you will need to write a custom xml serializer. I would caution against that beacuse the .NET XmlSerializer does a lot of special handling for you. Can you describe why you need things in the order you mention?

Answer 4

Technically, from a pure xml perspective, I would say that this is probably a bad thing to want to do.

.NET hides much of the complexity of things like XmlSerialization - in this case, it hides the schema to which your serialized xml should conform.

The inferred schema will use sequence elements to describe the base type, and the extension types. This requires strict ordering -- even if the Deserializer is less strict and accepts out of order elements.

In xml schemas, when defining extension types, the additional elements from the child class must come after the elements from the base class.

you would essentially have a schema that looks something like (xml-y tags removed for clarity)

base
  sequence
    prop1
    prop3

derived1 extends base
  sequence
    <empty>

derived2 extends base
  sequence
    prop2

There's no way to stick a placeholder in between prop1 and prop3 to indicate where the properties from the derived xml can go.

In the end, you have a mismatch between your data format and your business object. Probably your best alternative is to define an object to deal with your xml serialization.

For example

[XmlRoot("Object")
public class SerializableObjectForPersistance
{
    [XmlElement(Order = 1)]
    public bool Property1 { get; set; }

    [XmlElement(Order = 2, IsNullable=true)]
    public bool Property2 { get; set; }

    [XmlElement(Order = 3)]
    public bool Property3 { get; set; }
}

This separates your xml serialization code from your object model. Copy all the values from SerializableObject1 or SerializableObject2 to SerializableObjectForPersistance, and then serialize it.

Essentially, if you want such specific control over the format of your serialized xml that doesn't quite jive with the expectations xml serialization framework, you need to decouple your business object design (inheritance structure in this case) and the responsibility for serialization of that business object.

Answer 5

EDIT: This approach doesn't work. I've left the post in so that people can avoid this line of thinking.

The serializer acts recursively. There's a benefit to this; on deserialization, the deserialization process can read the base class, then the derived class. This means that a property on the derived class isn't set before the properties on the base, which could lead to problems.

If it really matters (and I'm not sure why it's important to get these in order) then you can try this --

1) make the base class' Property1 and Property3 virtual. 2) override them with trivial properties in your derived class. Eg

public class SerializableBase
{
    [XmlElement(Order = 1)]
    public virtual bool Property1 { get; set;}

    [XmlElement(Order = 3)]
    public virtual bool Property3 { get; set;}
}

[XmlRoot("Object")]
public class SerializableObject1 : SerializableBase
{
}

[XmlRoot("Object")]
public class SerializableObject2 : SerializableBase
{
    [XmlElement(Order = 1)]
    public override bool Property1 
    { 
      get { return base.Property1; }
      set { base.Property1 = value; }
    }

    [XmlElement(Order = 2)]
    public bool Property2 { get; set;}

    [XmlElement(Order = 3)]
    public override bool Property3
    { 
      get { return base.Property3; }
      set { base.Property3 = value; }
    }

}

This puts a concrete implementtion of the property on the most derived class, and the order should be respected.

Answer 6

I know this question has expired; however, here is a solution for this problem:

The name of the method should always begin with ShouldSerialize and then end with the property name. Then you simply need to return a boolean based on whatever conditional you want, as to whether to serialize the value or not.

public class SerializableBase
{
    public bool Property1 { get; set;}
    public bool Property2 { get; set;}
    public bool Property3 { get; set;}

    public virtual bool ShouldSerializeProperty2 { get { return false; } }
}

[XmlRoot("Object")]
public class SerializableObject1 : SerializableBase
{        
}

[XmlRoot("Object")]
public class SerializableObject2 : SerializableBase
{
    public override bool ShouldSerializeProperty2 { get { return true; } }
}

The outcome when using SerializableObject2: ~

<Object>
    <Property1></Property1>
    <Property2></Property2>
    <Property3></Property3>
</Object>

The outcome when using SerializableObject1: ~

<Object>
    <Property1></Property1>
    <Property3></Property3>
</Object>

Hope this helps many others!