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

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

I am curious why those methods do not type their return values as IList<TSource> and IDictionary<TKey, TSource> respectively. This seems even odder because ToLookup<TSource, TKey> types its return value as an interface instead of an actual implementation.

Looking at the source of those extension methods using dotPeek or other decompiler, we see the following implementation (showing ToList() because it is shorter):

public static List<TSource> ToList<TSource>(this IEnumerable<TSource> source) { 
   if (source == null) throw Error.ArgumentNull("source");
   return new List<TSource>(source); 
}

So why does this method type its return value as a specific implementation of the interface and not the interface itself? The only change would be the return type.

I am curious because the IEnumerable<> extensions are very consistent in their signatures, except for those two cases. I always thought it to be a bit strange.

Additionally, to make things even more confusing, the documentation for ToLookup() states:

> Creates a Lookup from an IEnumerable according to a > specified key selector function.

but the return type is ILookup<TKey, TElement>.

In Edulinq, Jon Skeet mentions that the return type is List<T> instead of IList<T>, but does not touch the subject further.
Extensive searching has yielded no answer, so here I ask you:

Is there any design decision behind not typing the return values as interfaces, or is it just happenstance?

Returning List<T> has the advantage that those methods of List<T> that are not part of IList<T> are easily used. There are a lot of things you can do with a List<T> that you cannot do with a IList<T>.

In contrast, Lookup<TKey, TElement> has only one available method that ILookup<TKey, TElement> does not have (ApplyResultSelector), and you probably would not end up using that anyway.

These kind of decisions may feel arbitrary but I guess that ToList() returns List<T> rather than an interface because List<T> both implements IList<T> but it adds other members not present in a regular IList<T>-typed object.

For example, AddRange().

See what IList<T> should implement (http://msdn.microsoft.com/en-us/library/5y536ey6.aspx):

public interface IList<T> : ICollection<T>, 
	IEnumerable<T>, IEnumerable

And List<T> (http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx):

public class List<T> : IList<T>, ICollection<T>, 
	IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, 
	IEnumerable

Maybe your own code doesn't require IReadOnlyList<T>, IReadOnlyCollection<T> or ICollection, but other components on .NET Framework and other products may rely on a more specialized list object and that's why .NET dev team decided to do not return an interface.

Don't feel always return an interface is the best practice. It's if your code or third-party ones require such encapsulation.

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.

In my opinion returning a List<T> is justified by the fact that the method name says ToList. Otherwise it would have to be named ToIList. It is the very purpose of this method to convert an unspecific IEnumerable<T> to the specific type List<T>.

If you had a method with an unspecific name like GetResults, then a return type like IList<T> or IEnumerable<T> would seem appropriate to me.

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If you look at the implementation of the Lookup<TKey, TElement> class with reflector, you'll see a lot of internal members, that are only accessible to LINQ itself. There is no public constructor and Lookup objects are immutable. Therefore there would be no advantage in exposing Lookup directly.

Lookup<TKey, TElement> class seems to be kind of LINQ-internal and is not meant for public use.

I believe that the decision to return a List<> instead of an IList<> is that one of the more common use cases for calling ToList is to force immediate evaluation of the entire list. By returning a List<> this is guaranteed. With an IList<> the implementation can still be lazy, which would defeat the "primary" purpose of the call.

This is one of the common things that programmers have difficulty understanding around the use of interfaces and concrete types.

Returning a concrete List<T> that implements IList<T> only gives the method consumer more information. Here is what the List object implements (via MSDN):

[SerializableAttribute]
public class List<T> : IList<T>, ICollection<T>, IList, ICollection, 
    IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable

Returning as a List<T> gives us the ability to call members on all of these interfaces in addition to List<T> itself. For example we could only use List.BinarySearch(T) on a List<T>, as it exists in List<T> but not in IList<T>.

In general to maximize flexibility of our methods, we should take the most abstract types as parameters (ie. only the things we're going to use) and return the least abstract type possible (to allow a more functional return object).

In general when you call ToList() on a method you're looking for a concrete type otherwise the item could stay as type IEnumerable. You don't need to convert to a List unless you're doing something that requires a concrete list.

The short answer is that in general returning the most specific type available is recommended by the authoritative Framework Design Guidelines. (sorry I don't have a citation on hand, but I remember this clearly since it stuck out in contrast to the Java community guidelines which prefer the opposite).

This makes sense to me. You can always do e.g. IList<int> list = x.ToList(), only the library author needs to be concerned with being able to support the concrete return type.

ToLookup<T> is the unique one in the crowd. But perfectly within the guidelines: it is the most specific type available that the library authors are willing to support (as others have pointed out, the concrete Lookup<T> type appears to be more of an internal type not meant for public use).

Because List<T> actually implements a range of interfaces, not just IList:

public class List<T> : IList<T>, ICollection<T>, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T>, IEnumerable<T>, IEnumerable{

}

Each of those interfaces define a range of features which the List must conform. Picking one particular one, would render bulk of the implementation unusable.

If you do want to return IList, nothing stops you from having your own simple wrapper:

public static IList<TSource> ToIList<TSource>(this IEnumerable<TSource> source)
{
    if (source == null) throw new ArgumentNullException(source);
    return source.ToList();
}

If a function returns a newly-constructed immutable object, the caller should generally not care about the precise type returned provided it is capable of holding the actual data that it contains. For example, a function that is supposed to return an IImmutableMatrix might normally return an ImmutableArrayMatrix backed by a privately-held array, but if all the cells hold zeroes it might instead return an ZeroMatrix, backed only by Width and Height fields (with a getter that simply returns zero all the time). The caller wouldn't care whether it was given an ImmutableArrayMatrix matrix or a ZeroMatrix; both types would would allow all of their cells to be read, and guarantee their values would never change, and that's what the caller would care about.

On the other hand, functions that return newly-constructed objects that allow open-ended mutation should generally return the precise type the caller is going to expect. Unless there will be a means by which the caller can request different return types (e.g. by calling ToyotaFactory.Build("Corolla") versus ToyotaFactory.Build("Prius")) there's no reason for the declared return type to be anything else. While factories that return immutable data-holding objects can select a type based on the data to be contained, factories that return freely-mutable types will have no way of knowing what data may be put into them. If different callers will have different needs (e.g. returning to the extant example, some callers' needs would be met with an array, while others' would not) they should be given a choice of factory methods.

BTW, something like IEnumerator<T>.GetEnumerator() is a bit of a special case. The returned object will almost always be mutable, but only in a very highly-constrained fashion; indeed, it is expected that the returned object regardless of its type will have exactly one piece of mutable state: its position in the enumeration sequence. Although an IEnumerator<T> is expected to be mutable, the portions of its state which would vary in derived-class implementations are not.