using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var l = new List<string>();

        dynamic s = "s";

        l.Add(s);

        var c = (ICollection<string>)l;

        c.Add(s);

        var i = (IList<string>)l;

        i.Add("s");

        i.Add(s);

    }

}