5

Suppose that I define two interfaces below:

public interface IReader
{
    void Read(string bookName);
}

public interface IWriter
{
    void Write(string bookName);
}

Now I want to implement IReader interface like this:

 public class Reader : IReader
{
    private IEnumerable<string> _books;

    public Reader(IEnumerable<string> books)
    {
        _books = books; // List of books is given through constructor
    }

    public void Read(string bookName)
    {
        if (_books.Contains(bookName))
        {
            // Reading here
        }
        else
        {
            throw new InvalidOperationException("Given book name was not found in the library.");
        }
    }
}

And now I want to define a higher-level class which implements both the two interfaces:

public class Person : IReader, IWriter
{
    // This is the data I want to share with IReader argument of constructor
    private IList<string> _books = new List<string>();
    private IReader _reader;

    public Person(IReader reader)
    {
        _reader = reader; // How do I share books with this object?
    }

    public void Read(string bookName)
    {
        _reader.Read(bookName);
    }

    public void Write(string bookName)
    {
        _books.Add(bookName);
    }
}

As you noticed I used composition to implement the IReader interface. I need to inject an instance of Reader to the Person class but I also need to share my Person's books data with the Reader instance. This is the problem I want to solve.

So far we didn't provide a way to share the list of books with the IReader interface. A few options come to my mind:

First option: Create an IReaderFactory interface and its implementation which creates an instance like this:

public interface IReaderFactory
{
    IReader Create(IEnumerable<string> books);
}

public class ReaderFactory : IReaderFactory
{
    public IReader Create(IEnumerable<string> books)
    {
        return new Reader(books);
    }
}

This is all good, now I can inject IReaderFactory to my Person constructor instead of IReader. The downside is that it requires 2 extra definitions IReaderFactory, and ReaderFactory.

Second option: Add another method to IReader interface like this:

void Initialize(IEnumreable<string> books);

This works too, in the Person's constructor I can call this method and pass my books to it. But it doesn't seem elegant to me, because now I have to remember calling this method, and this method can be called many times, which is something we don't want.

I don't want to use a DI container and resolve an instance of IReader inside Person's constructor because it hides the dependencies of the Person class.

Now, what is the best way to go in this kind of situation without violating Dependency Inversion Principle ?

  • What is the point in passing around a list of books if you can't see what is in the list? Even if you ignore the Person class, you have the same problem with your Reader class. How is the user of Reader supposed to know which books to tell the Reader to Read? If you can answer that for Reader then it is the same answer for Person. – Dunk Mar 16 '15 at 21:41
  • @Dunk, actually it is just an example to demonstrate my problem. Suppose I don't want to reveal the book collection to the user of the class. – Mert Akcakaya Mar 17 '15 at 7:24
3

DIP states that Higher level module should depend on Lower level module over abstraction and All variable implementations require Factory Method.Ref: DIP

So, creating a Factory Class definitely a good option to resolve your problem.

In addition, you could inject both IReader and Iwriter to your Person class.

public class Person : IReader, IWriter
{
    private IReader _reader;
    private IWriter _writer;

    public Person(IReader reader, IWriter writer)
    {
       _reader = reader;
       _writer = writer;
    }

    public void Read(string bookName)
    {
       _reader.Read(bookName);
    }

    public void Write(string bookName)
    {
       _writer.Write(bookName);
    }
}

For creating a Person object you can use Factory pattern.

public interface IPersonFactory
{
  Person Create(IEnumerable<string> books);
}

public class PersonFactory : IPersonFactory
{
  public Person Create(IEnumerable<string> books)
  {
    IWriter = new Writer(books); // if needed use factory class here
    IReader = new Reader(books); // if needed use factory class here
    return new Person(IReader, IWriter);
  }
}
  • Oh, it makes sense to create factories for higher-level classes, so the number of factories will be less compared to creating a factory for each lower-level classes. This way all the higher-level state can be shared among all lower-level classes too. – Mert Akcakaya Mar 17 '15 at 7:25
1

A third option might involve creating a class to manage the state of your books, then sharing an instance of the class with your Person and IReader implementations. This is conceptually similar to having a book repository or cache. Option 1 or my suggestion are probably the cleaner solutions.

Can you provide more information about the overall problem you are trying to model? I have a feeling that it can be modeled slightly differently and your issue will just go away.

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