.NET Serialization Ordering

Question

Hi,

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 what I have setup: ~

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

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

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

[XmlRoot("Object")]
public class SerializableObject2 : SerializableBase
{
    [XmlElement(Order = 2)]
    public bool Property2 { get; set;}
}

The outcome I want is as follows: ~

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

However I am getting an outcome of: ~

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

Does anyone know if it is possible or of any alternative?

Thanks

Answer 1

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 2

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 3

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.