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copied!<p>But what if you were building a collection yourself?</p> <p>In general, iterators can be used to <strong>lazily generate a sequence of objects</strong>. For example <code>Enumerable.Range</code> method does not have any kind of collection internally. It just generates the next number <strong>on demand</strong>. There are many uses to this lazy sequence generation using a state machine. Most of them are covered under <strong>functional programming concepts</strong>.</p> <p>In my opinion, if you are looking at iterators just as a way to enumerate through a collection (it's just one of the simplest use cases), you're going the wrong way. As I said, iterators are means for returning sequences. The sequence might even be <strong>infinite</strong>. There would be no way to return a list with infinite length and use the first 100 items. It <strong>has</strong> to be lazy sometimes. <strong>Returning a collection is considerably different from returning a collection generator</strong> (which is what an iterator is). It's comparing apples to oranges.</p> <p>Hypothetical example:</p> <pre><code>static IEnumerable<int> GetPrimeNumbers() { for (int num = 2; ; ++num) if (IsPrime(num)) yield return num; } static void Main() { foreach (var i in GetPrimeNumbers()) if (i < 10000) Console.WriteLine(i); else break; } </code></pre> <p>This example prints prime numbers less than 10000. You can easily change it to print numbers less than a million without touching the prime number generation algorithm at all. In this example, you can't return a list of all prime numbers because the sequence is infinite and the consumer doesn't even know how many items it wants from the start.</p>

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