Why are there no real immutable collections in C #?

I am currently learning C #, and I have a situation where I have a class containing ISet. I do not want clients to directly change this set, but most clients only add and delete, and I provide access through my class for this.

However, I have one client who wants to know more about this kit and its contents. I really don't want to mess up the wrapper class itself with a lot of methods for this single client, so I would prefer it to be able to return the set itself in an immutable way.

I found that I could not - well, actually. The only options I have are the following:

• Returns IEnumerable (No: restrictive functionality);
• ReadOnlyCollection (No: this is a LIST);
• Return a copy (No: IMHO's poor form, allows clients to modify the returned collection, possibly without suspecting that it is not going to modify the real object, and also has performance overhead);
• Implement my own ReadOnlySet (no: I would have to extract it from ISet and, therefore, I would need to implement mutators, perhaps by throwing exceptions, I would rather compile temporary errors - not runtime).

Am I missing something? Am I foolish? Is my only way to provide a complete set of accessories on my wrapper? Am I wrong in my initial intention to keep the shell clean for the vast majority of customers?

So, two questions:

• Why is there no standard C # interface for the collection? Does this sound like a pretty reasonable requirement?

• ReadOnlyCollection ReadOnlyCollection, ReadOnlyList? , , ( ).