There are times when it’s useful to be able to resolve two different interfaces to the same type when using Castle Windsor for dependency injection. Given a class called MyClass which implements two interfaces, IOne and ITwo, the following code will achieve this:

container.Register(
                Component.For<IOne, ITwo>()
                .ImplementedBy(typeof(MyClass))
                .LifeStyle.Singleton);

This can be simply demonstrated using the following classes:

namespace TwoInterfaces
{
    public interface IOne
    {
        string GetMessage();
    }

    public interface ITwo
    {
        string GetMessage();
    }

    public class MyClass : IOne, ITwo
    {
        public string GetMessage()
        {
            return "Hello World!";
        }
    }
}

…in a console application. Running the following program class:

using System;

namespace TwoInterfaces
{
    public static class Program
    {
        public static void Main(string[] args)
        {
            var injector = Injector.Instance;

            var one = injector.Resolve<IOne>();
            var two = injector.Resolve<ITwo>();

            Console.WriteLine(one.GetMessage());
            Console.WriteLine(two.GetMessage());
            Console.WriteLine(one.Equals(two));

            Console.WriteLine("Press RETURN to exit...");
            Console.ReadLine();
        }
    }
}

…yields:

…as expected. My post entitled Basic dependency injection with Castle Windsor explains how to set up a fully working Castle Windsor injector in more detail.