Why is indexing used in in the ICollection<T> Interface implementation example in Microsoft's documentation, if you cannot use it?

Question

I'm trying to understand how to implement generic collections and the IEnumerator interface; I'm using the Documentation provided to do so.

In the given example the enumerator's method MoveNext() is implemented as follows:

public bool MoveNext()
{
    //Avoids going beyond the end of the collection.
    if (++curIndex >= _collection.Count)
    {
        return false;
    }
    else
    {
        // Set current box to next item in collection.
        curBox = _collection[curIndex];
    }
    return true;
}

curIndex is used to as index for BoxCollection, which implements ICollection. If I try to do the same I get "Cannot apply indexing with [] to an expression of type 'System.Collections.Generic.ICollection...".

Is the documentation wrong, or is it me not doing something correctly?

Answer 1

BoxCollection itself implements the indexer:

public Box this[int index]
{
    get { return (Box)innerCol[index]; }
    set { innerCol[index] = value; }
}

(line 129-133 of the sample you linked to)

You're right that you can't use the indexer on a class that implements ICollection<T> - unless that class also implements an indexer.