Immutability of structs [duplicate]

Question

Possible Duplicate:
Why are mutable structs evil?

I read it in lots of places including here that it's better to make structs as immutable.

What's the reason behind this? I see lots of Microsoft-created structs that are mutable, like the ones in xna. Probably there are many more in the BCL.

What are the pros and cons of not following this guideline?

Answer 1

I use mutable structs often in my (performance critical) project - and I don't run into problems, because I understand the implications of copy semantics. As far as I can tell, the main reason people advocate immutable structs is so that people who don't understand the implications can't get themselves in trouble.

That's not such a terrible thing - but we're in danger now of it becoming "the gospel truth", when in fact there are times where it is legitimately the best option to make a struct mutable. Like with all things, there are exceptions to the rule.

Answer 2

The technical reason is that mutable structs appear to be able to do things that they don't actually do. Since the design-time semantics are the same as reference types, this becomes confusing for developers. This code:

public void DoSomething(MySomething something)
{
    something.Property = 10;
}

Behaves quite differently depending on if MySomething is a struct or a class. To me, this is a compelling, but not the most compelling reason. If you look at DDD's Value Object, you can see the connection to how structs should be treated. A value object in DDD can be best represented as a value type in .Net (and therefore a struct). Because it has no identity, it can't change.

Think of this in terms of something like your address. You can "change" your address, but the address itself hasn't changed. In fact, you have a new address assigned to you. Conceptually, this works, because if you actually changed your address, your roommates would have to move too.

Answer 3

Structs should represent values. Values do not change. The number 12 is eternal.

However, consider:

Foo foo = new Foo(); // a mutable struct
foo.Bar = 27;
Foo foo2 = foo;
foo2.Bar = 55;

Now foo.Bar and foo2.Bar is different, which is often unexpected. Especially in the scenarios like properties (fortunately the compiler detect this). But also collections etc; how do you ever mutate them sensibly?

Data loss is far too easy with mutable structs.