A mutable collection is not a subtype of an immutable collection. Instead, immutable and immutable collections are sibling descendants of readable collections. Unfortunately, the concepts of "readable", "read-only", and "immutable" seem to get blurred together, even though they mean three different things.

• A readable collection base class or interface type promises that one may read items, and does not provide any direct means of modifying the collection, but does not guarantee that code receiving the reference cannot cast or manipulate it in such a way as to permit modification.

• A read-only collection interface doesn't include any new members, but should only be implemented by a class which promises that there is no way to manipulate a reference to it in such a way as to mutate the collection nor receive a reference to something that could do so. It does not, however, promise that the collection won't be modified by something else which has a reference to the internals. Note that a read-only collection interface may not be able to prevent implementation by mutable classes, but can specify that any any implementation, or class derived from an implementation, which allows mutation shall be considered "legitimate" implementation or derivative of an implementation.

• An immutable collection is one which will always hold the same data as long as any reference to it exists. Any implementation of an immutable interface which does not always return the same data in response to a particular request is broken.

It is sometimes useful to have strongly-associated mutable and immutable collection types which both implement or derive from the same "readable" type, and to have the readable type include AsImmutable, AsMutable, and AsNewMutable methods. Such a design can allow code which wants to persist the data in a collection to call AsImmutable; that method will make a defensive copy if the collection is mutable, but skip the copy if it's already immutable.

A mutable collection is not a subtype of an immutable collection. Instead, mutable and immutable collections are sibling descendants of readable collections. Unfortunately, the concepts of "readable", "read-only", and "immutable" seem to get blurred together, even though they mean three different things.

• A readable collection base class or interface type promises that one may read items, and does not provide any direct means of modifying the collection, but does not guarantee that code receiving the reference cannot cast or manipulate it in such a way as to permit modification.

• A read-only collection interface doesn't include any new members, but should only be implemented by a class which promises that there is no way to manipulate a reference to it in such a way as to mutate the collection nor receive a reference to something that could do so. It does not, however, promise that the collection won't be modified by something else which has a reference to the internals. Note that a read-only collection interface may not be able to prevent implementation by mutable classes, but can specify that any any implementation, or class derived from an implementation, which allows mutation shall be considered an "illegitimate" implementation or derivative of an implementation.

• An immutable collection is one which will always hold the same data as long as any reference to it exists. Any implementation of an immutable interface which does not always return the same data in response to a particular request is broken.

It is sometimes useful to have strongly-associated mutable and immutable collection types which both implement or derive from the same "readable" type, and to have the readable type include AsImmutable, AsMutable, and AsNewMutable methods. Such a design can allow code which wants to persist the data in a collection to call AsImmutable; that method will make a defensive copy if the collection is mutable, but skip the copy if it's already immutable.