c# - Is there an easy way to make an immutable version of a class?

Question

Is there an easy way to make an instance immutable?

Let's do an example, I have a class holding a lots of data fields (only data, no behavior):

class MyObject
{
    // lots of fields painful to initialize all at once
    // so we make fields mutable :

    public String Title { get; set; }
    public String Author { get; set; }

    // ...
}

Example of creation:

MyObject CreationExample(String someParameters)
{
    var obj = new MyObject
    {
        Title = "foo"
        // lots of fields initialization
    };

    // even more fields initialization
    obj.Author = "bar";

    return obj;
}

But now that I have fully created my object, I don't want the object to be mutable anymore (because the data consumer will never need to change the state), so I would like something like that List.AsReadOnly:

var immutableObj = obj.AsReadOnly();

But if I want this behavior, I need to make another class that have exactly the same fields but without setter.

So is there any automatic way to generate this immutable class ? Or another way to allow mutability during creation but immutable once initialized ?

I know that fields can be marked as "readonly", but the object will be initialized outside of the class, and passing all fields as constructor parameters seems like a bad idea (too much parameters).

Answer 1

No, there is no easy way to make any type immutable, especially not if you want "deep" immutability (i.e. where no mutable object can be reached through the immutable object). You will have to explicitly design your types to be immutable. The usual mechanisms to make types immutable are these:

• Declare (property-backing) fields readonly. (Or, starting with C# 6 / Visual Studio 2015, use read-only auto-implemented properties.)

• Don't expose property setters, only getters.

• In order to initialize (property-backing) fields, you must initialize them in the constructor. Therefore, pass the (property) values to the constructor.

• Don't expose mutable objects, such as collections based on mutable-by-default types (like T[], List<T>, Dictionary<TKey,TValue>, etc.).

  If you need to expose collections, either return them in a wrapper that prevents modification (e.g. .AsReadOnly()), or at the very least return a fresh copy of the internal collection.

• Use the Builder pattern. The following example is too trivial to do the pattern justice, because it's usually recommended in cases where non-trivial object graphs need to be created; nevertheless, the basic idea is something like this:

  class FooBuilder // mutable version used to prepare immutable objects
  {
      public int X { get; set; }
      public List<string> Ys { get; set; }
      public Foo Build()
      {
          return new Foo(x, ys);
      }
  }
  
  class Foo // immutable version
  {
      public Foo(int x, List<string> ys)
      {
          this.x = x;
          this.ys = new List<string>(ys); // create a copy, don't use the original
      }                                   // since that is beyond our control
      private readonly int x;
      private readonly List<string> ys;
      …
  }