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copied!<p>Yield is useful because it saves you space. Most optimizations in programming makes a trade off between space (disk, memory, networking) and processing. Yield as a programming construct allows you to iterate over a collection many times in sequence without needing a separate copy of the collection for each iteration.</p> <p>consider this example:</p> <pre><code>static IEnumerable<Person> GetAllPeople() { return new List<Person>() { new Person() { Name = "George", Surname = "Bush", City = "Washington" }, new Person() { Name = "Abraham", Surname = "Lincoln", City = "Washington" }, new Person() { Name = "Joe", Surname = "Average", City = "New York" } }; } static IEnumerable<Person> GetPeopleFrom(this IEnumerable<Person> people, string where) { foreach (var person in people) { if (person.City == where) yield return person; } yield break; } static IEnumerable<Person> GetPeopleWithInitial(this IEnumerable<Person> people, string initial) { foreach (var person in people) { if (person.Name.StartsWith(initial)) yield return person; } yield break; } static void Main(string[] args) { var people = GetAllPeople(); foreach (var p in people.GetPeopleFrom("Washington")) { // do something with washingtonites } foreach (var p in people.GetPeopleWithInitial("G")) { // do something with people with initial G } foreach (var p in people.GetPeopleWithInitial("P").GetPeopleFrom("New York")) { // etc } } </code></pre> <p>(Obviously you are not required to use yield with extension methods, it just creates a powerful paradigm to think about data.)</p> <p>As you can see, if you have a lot of these "filter" methods (but it can be any kind of method that does some work on a list of people) you can chain many of them together without requiring extra storage space for each step. This is one way of raising the programming language (C#) up to express your solutions better.</p> <p>The first side-effect of yield is that it delays execution of the filtering logic until you actually require it. If you therefore create a variable of type IEnumerable<> (with yields) but never iterate through it, you never execute the logic or consume the space which is a powerful and free optimization.</p> <p>The other side-effect is that yield operates on the lowest common collection interface (IEnumerable<>) which enables the creation of library-like code with wide applicability.</p>

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