mapping.cs

namespace Scratch
{
    public class Tester
    {
        public class Taco<T>
        {
            public T Filling { get; set; }

            public Taco(T value)
            {
                Filling = value;
            }
        }

        public class Burrito<T>
        {
            public T Filling { get; set; }

            public Burrito(T value)
            {
                Filling = value;
            }
        }

        public class Tray<T>
        {
            public T Food { get; set; }

            public Tray(T value)
            {
                Food = value;
            }
        }

        #region Static Creators
        public static class Tray
        {
            public static Tray<T> Create<T>(T value)
            {
                return new Tray<T>(value);
            }
        }

        public static class Taco
        {
            public static Taco<T> Create<T>(T value)
            {
                return new Taco<T>(value);
            }
        }

        public static class Burrito
        {
            public static Burrito<T> Create<T>(T value)
            {
                return new Burrito<T>(value);
            }
        }
        #endregion

        // a function for converting from a taco to a burrito
        public static Burrito<T> ConvertTaco<T>(Taco<T> taco)
        {
            return Burrito.Create(taco.Filling);
        }

        // here is what I want... a higher order function for mapping Trays
        public static Tray<TOutput> ConvertTray<TInput, TOutput>(Tray<TInput> original, Func<TInput, TOutput> mapper)
        {
            return Tray.Create(mapper(original.Food));
        }

        public void Scratch()
        {
            var withTaco = Tray.Create(Taco.Create(new ChickenQueso()));

            // lame, one-off way to convert this tray
            var withBurrito1 = Tray.Create(ConvertTaco(withTaco.Food));

            // less-lame, way to convert the tray
            var withBurrito2 = ConvertTray(withTaco, ConvertTaco);

            // yes!
            var withBurrito3 = ConvertTray(withTaco, taco => Burrito.Create(taco.Filling));
        }
    }

    public class SteakFajita{}
    public class ChickenQueso{}
}

Fixed! Sweet. Thanks!