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copied!<p><code>foreach</code> <em>uses</em> the interfaces in many cases. You need the interfaces if you want to <em>implement</em> a sequence which <code>foreach</code> can then use. (Iterator blocks usually make this implementation task very simple though.)</p> <p>However, just <em>occasionally</em> 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:</p> <pre><code>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"); } } } </code></pre> <p>Likewise it can be useful to use the iterator if you want to treat (say) the first item differently to the rest:</p> <pre><code>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; } } </code></pre> <hr> <p>Now, if you're interested in the difference between <code>IEnumerator<T></code> and <code>IEnumerable<T></code>, you might want to think of it in database terms: think of <code>IEnumerable<T></code> as a table, and <code>IEnumerator<T></code> 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.</p> <p>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 <em>does</em> have that bit of state is helpful.</p>

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