Minimal Dependency Injection

Injection.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace ThinkingSites
{
    public class Injection
    {
        private static Dictionary<Type, Func<object>> Registry = new Dictionary<Type, Func<object>>();

        public static void Register<Tin,Tout>(Tout singleton = null) 
            where Tin : class 
            where Tout : class, new()
        {
            var tin = typeof(Tin);
            var tout = typeof(Tout);

            if (Registry.ContainsKey(typeof(Tin)))
            {
                throw new InvalidOperationException($"Type {tin.Name} has already been registered");
            }

            if (!tin.IsAssignableFrom(tout))
            {
                throw new InvalidOperationException($"Type {tout.Name} does not inherit from type {tin.Name}");
            }

            Func<object> func;
            if (singleton == null)
            {
                func = () => new Tout();
            }
            else
            {
                func = () => singleton;
            }
            Registry.Add(typeof(Tin), func);
        }

        public static Tout Get<Tout>() where Tout : class
        {
            if (Registry.ContainsKey(typeof(Tout)))
            {
                return (Tout)Registry[typeof(Tout)]();
            } else
            {
                throw new InvalidOperationException($"Type {typeof(Tout).Name} has not been registered");
            }
        }

        public static void Reset<Tout>()
        {
            Registry.Remove(typeof(Tout));
        }

        public static void ResetAll()
        {
            Registry.Clear();
        }
    }
}