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

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

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

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

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

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).