Just realized IEnumerable<T> is immutable, what are other most commonly used immutable generic interfaces? MSDN doesn't say [closed]

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Just realized IEnumerable is immutable, what are other commonly used immutable generic interfaces? MSDN doesn't say.

Or any good online resources?

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Apart from the fact that it doesn't make a whole lot of sense to refer to interfaces as being immutable, IEnumerable<T> isn't, for example:

List<string> myList = new List<string>();
myList.Add("Test"); // We just changed the state of myList, hence its not immutable

An example of an object that is immutable is String.

It doesn't make sense for an interface to be immutable as almost by definition an immutable object needs to be sealed, which an interface obviously cannot be.