Why create an IEnumerable?

Question

I don't understand why I'd create an IEnumerable. Or why it's important.

I'm looking at the example for IEnumerable: http://msdn.microsoft.com/en-us/library/system.collections.ienumerable.aspx

But I can basically do the same thing if I just went:

 List<Person> people = new List<Person>();

so what's IEnumerable good for? Can you give me a situation where I'd need to create a class that implements IEnumerable?

Answer 1

IEnumerable is an interface, it exposes certain things to the outside. While you are completely right, you could just use a List<T>, but List<T> is very deep in the inheritance tree. What exactly does a List<T>? It stores items, it offers certain methods to Add and Remove. Now, what if you only need the "item-keeping" feature of a List<T>? That's what an IEnumerable<T> is - an abstract way of saying "I want to get a list of items I can iterate over". A list is "I want to get a collection which I can modify, can access by index and iterate". List<T> offers a lot more functionality than IEnumerable<T> does, but it takes up more memory. So if a method is taking an IEnumerable<T>, it doesn't care what exactly it gets, as long as the object offers the possibilites of IEnumerable<T>.

Also, you don't have to create your own IEnumerable<T>, a List<T> IS an IEnumerable<T>!

Answer 2

Lists are, of course IEnumerable - As a general rule, you want to be specific on what you output but broad on what you accept as input eg:

You have a sub which loops through a list of objects and writes something to the console...

You could declare the parameter is as either IEnumerable<T> or IList<T> (or even List<T>). Since you don't need to add to the input list, all you actually need to do is enumerate - so use IEnumerable - then your method will also accept other types which implement IEnumerable including IQueryable, Linked Lists, etc...

You're making your methods more generic for no cost.

Answer 3

Today, you generally wouldn't use IEnumerable anymore unless you were supporting software on an older version of the framework. Today, you'd normally use IEnumerable<T>. Amongst other benefits, IEnumerable fully implements all of the LINQ operations/extensions so that you can easily query any List type that implements IEnumerable<T> using LINQ.

Additionally, it doesn't tie the consumer of your code to a particular collection implementation.