Why can I not assign a List of concrete types to a List of that concrete's interface?

Question

Why does this not compile?

public interface IConcrete { }

public class Concrete : IConcrete { }

public class Runner
{
    public static void Main()
    {
         


        

Answer 1

IList wouldn't work because IList isn't contravariant. It needs to be IEnumerable though again this only works in 4.0. You could also just use a ConvertAll with a lambda expression and that will work in 3.5