IList[string] instead of List[string]

Why, when I set a variable to a new instance of List[string], do I get an IList[string]? They aren't interchangeable and by giving me an IList[string] I have no access to any properties or methods not part of the IList interface such as AddRange which is a method of the List class but not part of the IList interface.

This seems to be a bug in your .NET implementation. What do I need to do to get a fully functional List[string] object?

Thanks.