IEnumerable , IEnumerator vs foreach, when to use what

Question

I was going through IEnumerable and IEnumerator , but could not get one point clearly..if we have foreach, then why do we need this two interfaces? Is there any scenario where we have to use interfaces.If yes, then can somebody explain with an example. Any suggestions and remarks are welcome. Thanks.

Answer 1

foreach uses the interfaces in many cases. You need the interfaces if you want to implement a sequence which foreach can then use. (Iterator blocks usually make this implementation task very simple though.)

However, just occasionally it can be useful to use the iterators directly. A good example is when trying to "pair up" two different sequences. For example, suppose you receive two sequences - one of names, one of ages, and you want to print the two together. You might write:

static void PrintNamesAndAges(IEnumerable<string> names, IEnumerable<int> ages)
{
    using (IEnumerator<int> ageIterator = ages.GetEnumerator())
    {
        foreach (string name in names)
        {
            if (!ageIterator.MoveNext())
            {
                throw new ArgumentException("Not enough ages");
            }
            Console.WriteLine("{0} is {1} years old", name, ageIterator.Current);
        }
        if (ageIterator.MoveNext())
        {
            throw new ArgumentException("Not enough names");
        }

    }
}

Likewise it can be useful to use the iterator if you want to treat (say) the first item differently to the rest:

public T Max<T>(IEnumerable<T> items)
{
    Comparer<T> comparer = Comparer<T>.Default;

    using (IEnumerator<T> iterator = items.GetEnumerator())
    {
        if (!iterator.MoveNext())
        {
            throw new InvalidOperationException("No elements");
        }
        T currentMax = iterator.Current;

        // Now we've got an initial value, loop over the rest
        while (iterator.MoveNext())
        {
            T candidate = iterator.Current;
            if (comparer.Compare(candidate, currentMax) > 0)
            {
                currentMax = candidate;
            }
        }
        return currentMax;
    }
}

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Now, if you're interested in the difference between IEnumerator<T> and IEnumerable<T>, you might want to think of it in database terms: think of IEnumerable<T> as a table, and IEnumerator<T> as a cursor. You can ask a table to give you a new cursor, and you can have multiple cursors over the same table at the same time.

It can take a while to really grok this difference, but just remembering that a list (or array, or whatever) doesn't have any concept of "where you are in the list" but an iterator over that list/array/whatever does have that bit of state is helpful.