gistfile1.cs

public class FooController : Controller{
  IService service;
  
  public FooController(IService service){
    this.service = service;
  }
  
  public ActionResult Bar(){
    service.DoStuff();
  }
}

public interface IService {
  void DoStuff();
}

public class MockService : IService{ 
  ...
}

public class RealService : IService {
  ...
}

public class ServiceSwitcher : IService {
  RealService realService;
  MockService mockService;
  
  public ServiceSwitcher(RealService realService, MockService mockService){
    this.realService = realService;
    this.mockService = mockService;
  }
  
  public void DoStuff(){
    if (Condition())
      mockService.DoStuff();
    else
      realService.DoStuff();
  }
}

void COntainerRegistration(){
  var cb = new ContainerBuilder();
  cb.RegisterType<RealService>().AsSelf();
  cb.RegisterType<MockService>().AsSelf();
  cb.RegisterType<ServiceSwitcher>().As<IService>();
  
}