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POHow to define IEnumerable behavior by contract?
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copied!<p>Consider this 2 methods that returns IEnumerable:</p> <pre><code> private IEnumerable<MyClass> GetYieldResult(int qtResult) { for (int i = 0; i < qtResult; i++) { count++; yield return new MyClass() { Id = i+1 }; } } private IEnumerable<MyClass> GetNonYieldResult(int qtResult) { var result = new List<MyClass>(); for (int i = 0; i < qtResult; i++) { count++; result.Add(new MyClass() { Id = i + 1 }); } return result; } </code></pre> <p>This code shows 2 different behaviors when calling some method of IEnumerable:</p> <pre><code> [TestMethod] public void Test1() { count = 0; IEnumerable<MyClass> yieldResult = GetYieldResult(1); var firstGet = yieldResult.First(); var secondGet = yieldResult.First(); Assert.AreEqual(1, firstGet.Id); Assert.AreEqual(1, secondGet.Id); Assert.AreEqual(2, count);//calling "First()" 2 times, yieldResult is created 2 times Assert.AreNotSame(firstGet, secondGet);//and created different instances of each list item } [TestMethod] public void Test2() { count = 0; IEnumerable<MyClass> yieldResult = GetNonYieldResult(1); var firstGet = yieldResult.First(); var secondGet = yieldResult.First(); Assert.AreEqual(1, firstGet.Id); Assert.AreEqual(1, secondGet.Id); Assert.AreEqual(1, count);//as expected, it creates only 1 result set Assert.AreSame(firstGet, secondGet);//and calling "First()" several times will always return same instance of MyClass } </code></pre> <p>It's simple to choose which behavior I want when my code returns IEnumerables, but how can I explicitly define that some method gets an IEnumerable as parameter that creates a single result set dispite of how many times it calls "First()" method.</p> <p>Of course, I don't want to force all itens to be created unnecessarily and I want to define the parameter as IEnumerable to say that no item will be included or removed from the collection.</p> <p>EDIT: Just to be clear, the question is not about how yield works or why IEnumerable can return different instances for each call. The question is how can I specify that a parameter should be a "search only" collection that returns same instances of MyClass when I call methods like "First()" or "Take(1)" several times.</p> <p>Any ideas?</p> <p>Thanks in advance!</p>

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