When dealing with series of objects, it is easy to think of them as lists, or arrays. That is, after all, the first collection types most people get acquainted with when becoming a programmer. And by all means, lists are very versatile and easy to employ in most situations. However, they often provide more functionality than you strictly need. Rather than considering collections of objects as lists, I find it helpful to think of them as sequences.
Since version 1.0, the .NET Framework has provided the
IEnumerable interface for iterating over a sequence of objects. These days, its generic cousin,
IEnumerable<T>, introduced in version 2.0, is often preferred. Initially, those interfaces existed mainly to support the
foreach statement. However, with the advent of LINQ to Objects,
IEnumerable got a morale boost. Suddenly, anyone could easily create advanced queries against any sequence of objects.
In my opinion, the main advantage of
IEnumerable is its stream-based nature. Like with streams, the first items of an
IEnumerable sequence can be made available for processing without having to collect every single item first. A sequences does not even have to be finite. Consider, for instance, the following code, returning an infinite sequence of all positive integers.
public static IEnumerable<int> EnumerateAllPositiveIntegers()
int integer = 0;
yield return integer;
yield return statement. This is a very handy shortcut which C# provides for creating
IEnumerable sequences. Simply
yield return every item you wish to include in the sequence. Those who are unfamiliar with such iterator blocks in C# may suspect that the code above will lead to an infinite loop. However, every
yield statement will lead to the enumerator’s
MoveNext method returning, giving control back to the client object. Of course, if the client iterates the sequence using a regular
foreach loop expecting the sequence to terminate, an infinite loop will in fact be the result.
The following code shows how the integer-generating method above can be used in a LINQ query, without risking an infinite loop.
IEnumerable<int> firstFiftyOddPositiveIntegers = EnumerateAllPositiveIntegers()
.Where(num => num % 2 == 1)
Rather than generating an infinite number of integers, consider an operation which needs to complete some time-consuming task for every value it returns. In such cases, exposing the results through an
IEnumerable sequence using
yield return will allow the client to process each value without having to wait for every value to be produced, collected and returned.
So, when are sequences preferable over lists and other collections? Obviously, if a series of items is potentially infinite, a sequence has to be used; it cannot be represented as a list or collection. Generally, sequences are intended for items to simply be iterated over, while lists are collections you can add items to and remove items from. My rule of thumb is to expose
IEnumerable sequences whenever feasible, relying on LINQ to convert the sequence into lists or arrays, should it be necessary.
By thinking in sequences, LINQ makes even more sense than before, and it also makes it easier to spot those situations where a sequence is not sufficient for the task.