Imagine the following scenario, we have a service class (MyService), and decide
to create simple caching mechanism, by creating a wrapper class (MyServiceWithCaching)

public interface IService
{
	List<string> MyMethod();
}

// Original service
public class MyService : IService
{
	public List<string> MyMethod() { ... }
}

// Service that supports simple caching
public class MyServiceWithCaching : IService
{
	IService _service;
	List<string> _cached;

	// NOTE: We want IService to be injected by IoC container
  	public MyServiceWithCaching(IService service)
  	{
		_service =service;
  	}

	public List<string> MyMethod()
	{
		if (_cached == null)
		{
			// call original when no cached data
			_cached = _service.MyMethod();
		}
		return _cached;
	}
}

The problem is how to tell StructureMap that when somebody asks for IMyService it should get MyServiceWithCaching which should be injected with original MyService class.

It took us some time to figure this out and here’s the solution:

public class IocRegistry : Registry
{
    protected override void configure()
    {
        this.BuildInstancesOf<IMyService>()
             .TheDefaultIs(
                Instance<IMyService>()
                    .UsingConcreteType<MyServiceWithCaching>()
                        .Child<IMyService>()
                            .IsConcreteType<MyService>()
            );

        base.configure();
    }
} ;