immutability

In the Design Guidelines for Developing Class Libraries , Microsoft say: Do not assign instances of mutable types to read-only fields. The objects created using a mutable type can be modified after they are created. For example, arrays and most collections are mutable types while Int32, Uri, and String are immutable types. For fields that hold a mutable reference type, the read-only modifier prevents the field value from being overwritten but does not protect the mutable type from modification. This simply restates the behaviour of readonly without explaining why it's bad to use readonly. The

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 C# usually means setting the initial values via the constructor. These values initialize readonly fields. But if you need to serialize a class like this to XML, using either the XmlSerializer or the DataContractSerializer, you must have a parameterless constructor. Does anyone have suggestions for how to work around this problem? Are there other forms of immutability that work better with serialization? EDIT: As @Todd pointed out

In a question about Best practices for C# pattern validation , the highest voted answer says: I tend to perform all of my validation in the constructor. This is a must because I almost always create immutable objects. How exactly do you create an immutable object in C#? Do you just use the readonly keyword? How exactly would this work if you want to validate in the constructor of your Entity Framework generated model class? Would it look like below? public partial readonly Person { public Person() } The interesting question here is your question from the comments: What kind of object would you

I've run into a problem where I have to store the initial values of a moment object but I'm having some trouble preventing my variable from changing along with the original object. Unfortunately Object.freeze() doesn't work, because moment.js returns an "Invalid date" error when I try to format that. There's a Moment.js plugin on NPM called frozen-moment - You could use moment().freeze() in place of Object.freeze(moment()) . Otherwise, vanilla Moment.js has a clone method that should help you avoid mutability problems, so you could do something like this: var a = moment(), b = a.clone(); // or

I was watching a presentation on Java, and at one point, the lecturer said: "Mutability is OK, sharing is nice, shared mutability is devil's work." What he was referring to is the following piece of code, which he considered an "extremely bad habit": //double the even values and put that into a list. List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 1, 2, 3, 4, 5); List<Integer> doubleOfEven = new ArrayList<>(); numbers.stream() .filter(e -> e % 2 == 0) .map(e -> e * 2) .forEach(e -> doubleOfEven.add(e)); He then proceeded writing the code that should be used, which is: List<Integer>

I'm rebuilding an old Java project in Javascript, and realized that there's no good way to do enums in JS. The best I can come up with is: const Colors = { RED: Symbol("red"), BLUE: Symbol("blue"), GREEN: Symbol("green") }; Object.freeze(Colors); The const keeps Colors from being reassigned, and freezing it prevents mutating the keys and values. I'm using Symbols so that Colors.RED is not equal to 0 , or anything else besides itself. Is there a problem with this formulation? Is there a better way? (I know this question is a bit of a repeat, but all the previous Q/As are quite old, and ES6