Why does IEnumerable.ToList() return List instead of IList?

Question

The extension method ToList() returns a List. Following the same pattern, ToDictionary() returns a Dictionary<

Answer 1

There are a number of advantages to just having a List over an IList. To begin with, List has methods that IList does not. You also know what the implementation is which allows you to reason about how it will behave. You know it can efficiently add to the end, but not the start, you know that it's indexer is very fast, etc.

You don't need to worry about your structure being changed to a LinkedList and wrecking the performance of your application. When it comes to data structures like this it really is important in quite a lot of contexts to know how your data structure is implemented, not just the contract that it follows. It's behavior that shouldn't ever change.

You also can't pass an IList to a method accepting a List, which is something that you see quite a lot of. ToList is frequently used because the person really needs an instance of List, to match a signature they can't control, and IList doesn't help with that.

Then we ask ourselves what advantages there are to returning IList. Well, we could possibly return some other implementation of a list, but as mentioned before this is likely to have very detrimental consequences, almost certainly much more than could possibly be gained from using any other type. It might give you warm fuzzies to be using an interface instead of an implementation, but even that is something I don't feel is a good mentality (in general or) in this context. As a rule returning an interface is generally not preferable to returning a concrete implementation. "Be liberal in what you accept and specific in what you provide." The parameters to your methods should, where possible, be interfaces defining the least amount of functionality you need to that your caller can pass in any implementation that does what you need of it, and you should provide as concrete of an implementation as the caller is "allowed" to see so that they can do as much with the result as that object is capable of. Passing an interface is restricting that value, which is only occasionally something that you want to do.

So now we move onto, "Why return ILookup and not Lookup?" Well, first off Lookup isn't a public class. There is no Lookup in System.Collections.*. The Lookup class that is exposed through LINQ exposes no constructors publicly. You're not able to use the class except through ToLookup. It also exposes no functionality that isn't already exposed through ILookup. In this particular case they designed the interface specifically around this exact method (ToLookup) and the Lookup class is a class specifically designed to implement that interface. Because of all of this virtually all of the points discussed about List just don't apply here. Would it have been a problem to return Lookup instead, no, not really. In this case it really just doesn't matter much at all either way.