Define your own IoC container

This is a simple IoC Container.

Last year I was assigned to develop an IoC container for R&D purposes which made me start working with IoC Containers. I share with you here part of it as I think it might help you a lot when you develop your own container.

The Inversion of Control (IoC) and Dependency Injection (DI) patterns are all about eliminating dependencies from your code. Dependency Injection is well-known as “Inversion of Control” whose principle is – “Don’t call us, we’ll call you”. So we can say that IoC is the principle whereas DI is the way of implementation. There are three popular ways of Inversion of Control namely:

1. Constructor Injection
2. Method Injection
3. Property Injection

I am not going to discuss about IoC or DI rather I want to show how to implement your own IoC Container. However if you want to know the details about IoC and DI, please have a look at this link.

Let's move to our code portion: 

public static class IoC
{
    private static readonly IDictionary<Type, Type> types = new Dictionary<Type, Type>();
    private static readonly IDictionary<Type, object> typeInstances = new Dictionary<Type, object>();
    public static void Register<TContract, TImplementation>()
    {
        types[typeof(TContract)] = typeof(TImplementation);
    }
    public static void Register<TContract, TImplementation>(TImplementation instance)
    {
        typeInstances[typeof(TContract)] = instance;
    }
    public static T Resolve<T>()
    {
        return (T)Resolve(typeof(T));
    }
    public static object Resolve(Type contract)
    {
        if (typeInstances.ContainsKey(contract))
        {
            return typeInstances[contract];
        }
        else
        {
            Type implementation = types[contract];
            ConstructorInfo constructor = implementation.GetConstructors()[0];
            ParameterInfo[] constructorParameters = constructor.GetParameters();
            if (constructorParameters.Length == 0)
            {
                return Activator.CreateInstance(implementation);
            }
            List<object> parameters = new List<object>(constructorParameters.Length);
            foreach (ParameterInfo parameterInfo in constructorParameters)
            {
                parameters.Add(Resolve(parameterInfo.ParameterType));
            }
            return constructor.Invoke(parameters.ToArray());
        }
    }
}

Now from your bootstrapper class, you need to register your class like:

Service svc= new Service ();
IoC.Register<IService , Service >(svc);

On subsequent classes, you just need to inject the Interface or class that you register like:

private IService currentSvc = null;
public LoginServices(IService  submittedService)
{
    this.currentSvc =submittedService;
}

Very very simple!!!! We love simple, don’t we?

Updated: 19 March2012

Example: Say this is my Interface,

public interface ILogger
{
    void Log(string submittedMsg);
}

The Logger class implements this interface.

public class Logger :ILogger
{
    public void Log(string submittedMsg)
    {
        if (!string.IsNullOrEmpty(submittedMsg))
        {
            Console.WriteLine(string.Format("[{0}] {1}\r\n", DateTime.Now, submittedMsg));
            Console.Read();
        }
    }
}

In this ConsoleLogger class, I inject the ILogger to its constructor,

public class ConsoleLogger
{
   private ILogger logger = null;

   public ConsoleLogger(ILogger submittedLogger)
   {
       this.logger = submittedLogger;
       this.DisplayMessage();
   }

   public void DisplayMessage()
   {
       logger.Log("Hi..See it is working!!");
   }
}

Register and Resolve options:

class Program
{
    static void Main(string[] args)
    {
        IoC.Register<ILogger, Logger>();

        IoC.Register<ConsoleLogger, ConsoleLogger>();
        ConsoleLogger objCOnsoleLogger=IoC.Resolve<ConsoleLogger>();       
    }
}