Immutability and XML Serialization

Question

I have several classes that are immutable once their initial values are set. Eric Lippert calls this write-once immutability.

Implementing write-once immutability in

Answer 1

I just had a look at the article you linked to. In his terminology, objects using readonly fields initialized in the constructor are called "write-only immutability".

"Popsicle immutability" is a bit different. Lippert gives two examples of where it would be useful: deserialization (the problem which you are trying to solve), and circular references where A and B need to be created independently but A needs have a reference to B, and B a reference to A.

The two more obvious ways to achieve this result have been mentioned here (as I was writing this, in fact). The pattern Thomas Levesque mentions is basically the "Builder" pattern. But it's rather unwieldly in this case because you need to not only go from Builder to Immutable, but also from Immutable to Builder so that the serialization/deserialization is symmetrical.

So the "lock" pattern, as mentioned by Marc Gravell, seems more useful. I'm not familiar with C# though, so I'm not sure how best to implement it. I guess probably a private property such as locked. Then all the getter methods need to explicitly check whether the object is locked (aka "frozen") yet. If so they should throw an exception (it's a runtime error; you cannot enforce popstick immutability at compile time).