If method returns interface type, why can't I pass the result to a concrete type?

Question

The question maybe a little confusing, but it's hard to make clear this question in a subject title.

I have method declared and implemented like this:

public IList<string> GetBookTitles()
{
    IList<string> bookTitles = new List<string>();
    // do something to populate the bookTitles list.
    return bookTitles;
}

Why can't I pass the result of this method to a List<string>? After all, List<string> is a kind of IList<string>.

Answer 1

Well, for starters, just look at the members of IList and compare it with List. List has methods that an IList doesn't. (List has a BinarySearch method that IList doesn't, just as a single example.)

Arrays also implement IList, as an example. An array however is not a List, so you can't, and shouldn't, be able to pass a string[] to a method that accepts a List<string>.

You have a few possible solutions. One would be to just change your method to return a List<string> rather than an IList<string> (that's what I'd suggest). If that's what you really need then you shouldn't be restricting the return type to IList<string>. Another (poorer) option would be to cast the result back to a List<string> before passing it to the next method, since you happen to know that it's what the underlying type really is.