The true definition of immutability?

Question

I am wondering how immutability is defined? If the values aren't exposed as public, so can't be modified, then it's enough?

Can the values be modified inside the type, not by the customer of the type?

Or can one only set them inside a constructor? If so, in the cases of double initialization (using the this keyword on structs, etc) is still ok for immutable types?

How can I guarantee that the type is 100% immutable?

Answer 1

If the values aren't exposed as public, so can't be modified, then it's enough?

No, because you need read access.

Can the values be modified inside the type, not by the customer of the type?

No, because that's still mutation.

Or can one only set them inside a constructor?

Ding ding ding! With the additional point that immutable types often have methods that construct and return new instances, and also often have extra constructors marked internal specifically for use by those methods.

How can I guarantee that the type is 100% immutable?

In .Net it's tricky to get a guarantee like this, because you can use reflection to modify (mutate) private members.

Answer 2

The previous posters have already stated that you should assign values to your fields in the constructor and then keep your hands off them. But that is sometimes easier said than done. Let's say that your immutable object exposes a property of the type List<string>. Is that list allowed to change? And if not, how will you control it?

Eric Lippert has written a series of posts in his blog about immutability in C# that you might find interesting: you find the first part here.