Why is indexing used in in the ICollection 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?

score 3 · Accepted Answer · answered Apr 06 '21 at 06:31

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.

score 1 · Answer 2 · answered Apr 06 '21 at 06:36

In the sample code in the documentation _collection is a BoxCollection, which is an ICollection also, but in that manifestation it’s typed as a BoxCollection and can therefore have indexing applied because BoxCollection implements a this[int] indexer property

Had the sample code declared _collection as some ICollection<T>, their code would get the same error yours does; in other words the indexability comes from their variable being an indexable type, which is unrelated to it also implementing ICollection (ICollection does not mandate provision of an indexer)

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

2 Answers2