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

You could try

public void DoStuffWithInterface(IList concrete) { }

but I think this only works in .NET 4.0.

If you wanna be dirty just do

public void DoStuffWithInterface(IList concrete) { }

and check to see if the objects coming out are concrete.