Immutable object (de)serialization

[vekipeki]

Hi!

I've been looking around for a simple example of the "right way" to implement immutable object xml serialization/deserialization.

The obvious problem is that the immutable object's properties are made read-only by omitting the setter, while private fields are made read-only by using the readonly keyword.

Since IXmlSerializable.ReadXml(System.Xml.XmlReader reader) requires the object inner state to be changed, I have to make private fields writable, which I would like to avoid. I've tried googling it but was unable to find the answer I was looking for.

Thanks a lot,

Veki

[vekipeki]

Just to make my question a bit clearer, one of the simple classes named UnitInfo is implemented like this:

class UnitInfo
{
   private readonly Unit _unit;   // Unit is enum
   public Unit { get { return _unit; }}
 
   private readonly UnitPrefix _unitPrefix;   // UnitPrefix is enum
   public UnitPrefix { get { return _unitPrefix; }}
 
   public UnitInfo(Unit unit, UnitPrefix unitPrefix)
   {
       _unit = unit; _unitPrefix = unitPrefix;
   }
}
 

Since it is used at many places in code, it was made immutable to make sure that no object containing a UnitInfo can corrupt other objects' UnitInfos, and since those UnitInfos are in 80% cases equal by value, this way they can safely reference the same instance.

But the problem is that I am not sure how to implement serialization/deserialization? XmlSerializer by itself will skip the readonly properties, and I cannot change the fields outside the constructor if they are also readonly.

I am pretty sure I am not the only one using immutable objects -- someone must have had the same problem before? :)

[vekipeki]

I got it at the end, so I will just post a quick answer if anyone comes across the same problem:

I created a base ImmutableObject class which implements IXmlSerializable, and then uses Reflection to serialize/deserialize private instance fields marked with a custom attribute (I named it [SerializeImmutableAs("NameOfElement")], because private fields usually have lower camel casing or who-knows-what-soft-of-casing. Since Reflection does not care about readonly attributes, it can change the state of the object outside the constructor.

Derived object don't have to implement anything, and all messing with Reflection is constrained to IXmlSerializable members in base class only.