如日志这种横切关注点，就不适合构造注入，而是适合公共属性注入。
public class PersonAppService{
public ILogger Logger { get; set; }
private IPersonRepository _personRepository;
public PersonAppService(IPersonRepository personRepository)
{
_personRepository = personRepository;
Logger = NullLogger.Instance;
}
public void CreatePerson(string name, int age)
{
Logger.Debug("Inserting a new person to database with name = " + name);
var person = new Person { Name = name, Age = age };
_personRepository.Insert(person);
Logger.Debug("Successfully inserted!");
}
}
