C# Online Compiler | .NET Fiddle

using System;
using Microsoft.Practices.Unity;

namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var container = new UnityContainer();

            container.RegisterType<IClient>(new InjectionFactory(c => new Decorator(c.Resolve<Client>())));
            container.RegisterType<ICounter, Decorator>();

            container.Resolve<Service>();

            Console.WriteLine(Decorator.NumberOfInstances);
        }
    }

    public class Client : IClient
    {
    }

    public class Decorator : IClient, ICounter
    {
        public static int NumberOfInstances { get; private set; }

        public Decorator(IClient client)
        {
            NumberOfInstances++;
        }        
    }

    public interface ICounter
    {
    }

    public interface IClient
    {
    }

    public class Service
    {
        public Service(IClient client, ICounter counter)
        {
        }    
    }
}

 
using System;
using Microsoft.Practices.Unity;
​
namespace ConsoleApplication1
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var container = new UnityContainer();
​
            container.RegisterType<IClient>(new InjectionFactory(c => new Decorator(c.Resolve<Client>())));
            container.RegisterType<ICounter, Decorator>();
​
            container.Resolve<Service>();
​
            Console.WriteLine(Decorator.NumberOfInstances);
        }
    }
​
    public class Client : IClient
    {
    }
​

Cached Result

Hello World