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

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Question Description

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

Practice As Follows

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…

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