How do I find out if a class is immutable in C#?

Question

How do I find out if a class is immutable in C#?

Answer 1

There is ImmutableObjectAttribute, but this is rarely used and poorly supported - and of course not enforced (you could mark a mutable object with [ImmutableObject(true)]. AFAIK, the only thing this this affects is the way the IDE handles attributes (i.e. to show / not-show the named properties options).

In reality, you would have to check the FieldInfo.IsInitOnly, but this only applies to truly 100% immutable types (assuming no reflection abuse, etc); it doesn't help with popsicle immutability, nor things that are immutable in practice, but not in their implementation; i.e. they can't be made to be publicly mutable, but in theory the object supports it.

A classic example here would be string... everyone "knows" that string is immutable... of course, StringBuilder does mutate a string under the bonnet. No, seriously...

It is so hard to define immutability given this, let alone robustly detect it...

Answer 2

Part of the problem is that "immutable" can have multiple meanings. Take, for example, ReadOnlyCollection<T>.

We tend to consider it to be immutable. But what if it's a ReadOnlyCollection<SomethingChangeable>? Also, since it's really just a wrapper around an IList I pass in to the constructor, what if I change the original IList?

A good approach might be to create an attribute with a name like ReadOnlyAttribute and mark classes you consider to be read-only with it. For classes you don't control, you can also maintain a list of known types that you consider to be immutable.

EDIT: For some good examples of different types of immutability, read this series of postings by Eric Lippert: http://blogs.msdn.com/ericlippert/archive/2007/11/13/immutability-in-c-part-one-kinds-of-immutability.aspx