Contract of ICollection<T>.IsReadOnly
.net, collections, msdn
Solution
Something else to consider...
Your collection is an array wrapper and it has some array-like semantics. ie, Items can't be inserted or removed but they can be modified.
Arrays return `false` for `IsReadOnly` and `true` for `IsFixedSize`.
I think I would probably implement `IList` in addition to `IList<T>` and then mimic the array behaviour, so far as `IsReadOnly` and `IsFixedSize` are concerned.
The key word in the remark from MSDN is the "or":
A collection that is read-only does not allow the addition, removal, or modification of elements after the collection is created.
Your collection does allow modification, so returning `true` for `IsReadOnly` would be breaking that contract, in my opinion.
Problem
I'm writing an array wrapper class that implements `IList<T>`. I’m unsure of what to return for `IList<T>.IsReadOnly` (inherited from `ICollection<T>`), though. My class disallows insertion and removal. It does allow modifying items via the `this[int].set` property. The MSDN states that A collection that is read-only does not allow the addition, removal, or modification of elements after the collection is created. For my class, this seems to imply that I have to return `true` but in my eyes this renders the property a bitcompletely useless: as far as I can see, the use of this method is as follows: The clients handle an arbitrary `IList` and need to insert an element into it, if at all possible. They can do this by just calling `Insert` and catching the resulting `NotSupportedException` – and for various reasons, this may not be desirable. So instead of provoking an exception, the clients can just test the `IsReadOnly` property beforehand. But the result of this property will be wrong because it mixes modifiability of the collection with modifiability of its contents – which are completely unrelated matters! To be sure, there’s the `IList.IsFixedSize` property but this is a separate type (`IList<T>` does not extend `IList`). What should I do? Also implement `IList` (I really don’t like this alternative)? Do something else?