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

Answer 1

One thing that I think might be missed in all these answers is that I think that an object can be considered immutable even if its internal state changes - as long as those internal changes are not visible to the 'client' code.

For example, the System.String class is immutable, but I think it would be permitted to cache the hash code for an instance so the hash is only calculated on the first call to GetHashCode(). Note that as far as I know, the System.String class does not do this, but I think it could and still be considered immutable. Of course any of these changes would have to be handled in a thread-safe manner (in keeping with the non-observable aspect of the changes).

To be honest though, I can't think of many reasons one might want or need this type of 'invisible mutability'.