Design pattern

namespace Coding.Exercise
  {
    public interface IValueContainer
    {
      
    }

    public class SingleValue : IValueContainer
    {
      public int Value;
    }

    public class ManyValues : List<int>, IValueContainer
    {
      
    }

    public static class ExtensionMethods
    {
      public static int Sum(this List<IValueContainer> containers)
      {
        int result = 0;
        foreach (var c in containers)
        foreach (var i in c)
          result += i;
        return result;
      }
    }
  }