SmallInjectorDemo/SmallInjector.cs

using System;
using System.Collections.Generic;

namespace SmallInjectorDemo
{
    /// <summary>
    /// A small dependency injector to demonstrate the pattern.
    /// </summary>
    public class SmallInjector
    {
        private readonly Dictionary<Type, RegisteredType> _container = new Dictionary<Type, RegisteredType>();

        /// <summary>
        /// Register types in the resolve container.
        /// </summary>
        /// <typeparam name="TService">Type of the service.</typeparam>
        /// <typeparam name="TInterface">Type of the interface of the service.</typeparam>
        /// <param name="isSingleton">True if the service should be singleton. False otherwise.</param>
        public void RegisterType<TService, TInterface>(bool isSingleton)
        {
            _container.Add(typeof(TInterface), new RegisteredType(typeof(TService), isSingleton));
        }

        /// <summary>
        /// Register types in the resolve container.
        /// </summary>
        /// <typeparam name="TService">Type of the service.</typeparam>
        public void RegisterType<TService>(bool isSingleton) => RegisterType<TService, TService>(isSingleton);

        /// <summary>
        /// Resolve service of specified type.
        /// </summary>
        /// <typeparam name="TService">Type of the service.</typeparam>
        /// <returns>A instance of the service.</returns>
        public TService Resolve<TService>() => (TService)Resolve(typeof(TService));

        /// <summary>
        /// Resolve service of specified type.
        /// </summary>
        /// <param name="serviceInterface">Type of the service.</param>
        /// <returns>A instance of the service.</returns>
        public object Resolve(Type serviceInterface)
        {
            if (!_container.TryGetValue(serviceInterface, out var registeredType))
            {
                throw new Exception("Can't resolve dependency " + serviceInterface);
            }

            if (registeredType.IsSingleton && registeredType.SingletonInstance != null)
                return registeredType.SingletonInstance;
            var constructor = registeredType.ConcreteType.GetConstructors()[0];
            var parameters = constructor.GetParameters();
            var constructorParameters = new object[parameters.Length];
            for (var i = 0; i < parameters.Length; i++)
            {
                constructorParameters[i] = Resolve(parameters[i].ParameterType);
            }

            if (registeredType.IsSingleton)
            {
                return registeredType.SingletonInstance ?? (registeredType.SingletonInstance = constructor.Invoke(constructorParameters));
            }
            return constructor.Invoke(constructorParameters);
        }

        private class RegisteredType
        {
            public RegisteredType(Type concreteType, bool isSingleton)
            {
                ConcreteType = concreteType;
                IsSingleton = isSingleton;
            }

            public readonly bool IsSingleton;
            public readonly Type ConcreteType;
            public object SingletonInstance { get; set; }
        }
    }
}