Is ReadOnlyCollection<char> immutable?

Question

With the code below i get a CA2104 (DoNotDeclareReadOnlyMutableReferenceTypes) warning

public readonly ReadOnlyCollection<char> IllegalChars;

with part of the error message

change the field to one that is an immutable reference type. If the reference type 'ReadOnlyCollection' is, in fact, immutable, exclude this message.

I am sure ReadOnlyCollection is immutable but my question is is there a type can i use to not have this message appear?

Answer 1

Yoy can rewrite the public field to a property:

public ReadOnlyCollection<char> IllegalChars { get; private set; }

or exclude the Code Analysis warning, because the collection can't be changed. The problem is that FxCop is unable to detect that the ReadOnlyCollection is actually immutable.