What's an Immutable Class?

The term immutable in OOP is used to be mean that once an object is created, its content cannot be changed; a class is immutable if its instances are immutable.

To create a object which is immutable you need to make sure that the following conditions are met:

1. The Class should be final (sealed)
2. The Properties of the class should be private and should not have any setter methods

The above mentioned are bare minimum conditions that needs to be satisfied to make the object immutable; to learn more on Immutability in C# you can see:

• A good introduction to immutable types by Patrick Smacchia: Immutable types: understand their benefits and use them
• Luca Bolognese on implementing immutable value objects:
  • Creating an immutable value object in C# - Part I - Using a class
  • Creating an immutable value object in C# - Part II - Making the class better
  • Creating an immutable value object in C# - Part III - Using a struct
  • Creating an immutable value object in C# - Part IV - A class with a special value
  • Creating an immutable value object in C# - Part V - Using a library
• Eric Lippert shows how to implement a few common data structures as immutable types:
  • Immutability in C# Part One: Kinds of Immutability
  • Immutability in C# Part Two: A Simple Immutable Stack
  • Immutability in C# Part Three: A Covariant Immutable Stack
  • Immutability in C# Part Four: An Immutable Queue
  • Immutability in C# Part Five: LOLZ!
  • Immutability in C# Part Six: A Simple Binary Tree
  • Immutability in C# Part Seven: More on Binary Trees
  • Immutability in C# Part Eight: Even More On Binary Trees
• Tips: Return read-only collection if you want to control its modification

How To Serialize an Immutable Class

Obviously you can't simply add the Serializable attribute to make an immutable class serializable 'cause its properties are readonly, so you need to implement explicitly the ISerializable interface to control the serialization/deserialization process; to expose the class through a Web Service, you must implement also the IXmlSerializable interface.

Maybe an example will be useful for you:

using System;
using System.Runtime.Serialization;
using System.Xml;
using System.Xml.Schema;
using System.Xml.Serialization;

[Serializable]
public sealed class Car : ISerializable, IXmlSerializable
{
    public Car() {}

    public Car(int id, string model, DateTime creationDate)
    {
        _id = id;
        _model = model;
        _creationDate = creationDate;
    }

    private int _id = -1;
    public int Id
    {
        get { return _id; }
    }

    private string _model = string.Empty;
    public string Model
    {
        get { return _model; }
    }

    private DateTime _creationDate = DateTime.MinValue;
    public DateTime CreationDate
    {
        get { return _creationDate; }
    }

    #region ISerializable Members

    private Car(SerializationInfo info, StreamingContext context)
    {
        _id = info.GetInt32("Id");
        _model = info.GetString("Model");
        _creationDate = info.GetDateTime("CreationDate");
    }

    public void GetObjectData(SerializationInfo info, StreamingContext context)
    {
        info.AddValue("Id", _id);
        info.AddValue("Model", _model);
        info.AddValue("CreationDate", _creationDate);
    }

    #endregion

    #region IXmlSerializable Members

    public XmlSchema GetSchema()
    {
        return null;
    }

    public void ReadXml(XmlReader reader)
    {
        while (reader.Read())
        {
            if (reader.NodeType == XmlNodeType.Element)
            {
                switch (reader.LocalName)
                {
                    case "Id":
                        _id = (reader.IsEmptyElement) ? -1 : reader.ReadElementContentAsInt();
                        break;
                    case "Model":
                        _model = (reader.IsEmptyElement) ? string.Empty : reader.ReadElementContentAsString();
                        break;
                    case "CreationDate":
                        _creationDate = (reader.IsEmptyElement) ? DateTime.MinValue : reader.ReadElementContentAsDateTime();
                        break;
                    default:
                        continue;
                }
            }
        }
    }

    public void WriteXml(XmlWriter writer)
    {
        writer.WriteElementString("Id", XmlConvert.ToString(_id));
        writer.WriteElementString("Model", _model);
        writer.WriteElementString("CreationDate", XmlConvert.ToString(_creationDate, XmlDateTimeSerializationMode.Utc));
    }

    #endregion

}

HTH