Help with Windsor and Repository and Unit of Work patterns

Question

I have these interfaces:

public interface IUnitOfWork
{
    IPersonRepository People { get; }
    IBookRepository Books { get; }
    int Commit();
}

public interface IBookRepository
{
    Book GetBookById(int id);
    IQueryable<Book> GetAllBooks();
}

public interface IPersonRepository
{
    Person GetPersonById(int id);
    IQueryable<Person> GetAllPeople();
}

I implement IUnitOfWork as follow:

public class SqlUnitOfWork : IUnitOfWork
{
    private readonly DbContext dbContext;

    public SqlUnitOfWork()
    {
        dbContext = new DbContext("name=SQLContainer");
    }

    public IPersonRepository People
    {
        get { return IoC.Container.Resolve<IPersonRepository>(new { DbContext = dbContext }); }
    }

    public IBookRepository Books
    {
        get { return IoC.Container.Resolve<IBookRepository>(new { DbContext = dbContext }); }
    }

    public int Commit()
    {
        return dbContext.SaveChanges();
    }
}

The implementations of IBookRepository and IPersonRepository uses a constructor that takes a DbContext as a parameter, and this DbContext is created in the SqlUnitOfWork (code above) and I pass this parameter using an overload of the Resolve method.

My question is, is this the right way to do it? Is this a good practice?

Thanks!

Answer 1

Using a DI Container as a Service Locator can hardly be said to be good practice. In addition to that, passing the DbContext to the container while resolving an interface is a Leaky Abstraction because it implies that you know something about the concrete implementation that you should not.

Instead I would recommend Constructor Injection, which would go something like this:

public class SqlUnitOfWork : IUnitOfWork
{
    private readonly DbContext dbContext;
    private readonly IPersonRepository personRepository;
    private readonly IBookRepository bookRepository;

    public SqlUnitOfWork(DbContext dbContext,
         IPersonRepository personRepository, IBookRepository bookRepository)
    {
        if (dbContext == null)
            throw new ArgumentNullException("dbContext");
        if (personRepository == null)
            throw new ArgumentNullException("personRepository");
        if (bookRepository = null)
            throw new ArgumentNullException("bookRepository");

        this.dbContext = dbContext;
        this.personRepository = personRepository;
        this.bookRepository = bookRepository;
    }

    public IPersonRepository People
    {
        get { return this.personRepository; }
    }

    public IBookRepository Books
    {
        get { return this.bookRepository; }
    }

    public int Commit()
    {
        return this.dbContext.SaveChanges();
    }
}

Even though there's no explicit sharing of DbContext, this can be configured through the container. Since the context of this question indicates that Castle Windsor is the container being used, the default lifetime is already Singleton, so you don't have to explicitly set this up. With Castle Windsor, the DbContext will automatically be shared between the SqlUnitOfWork class and both repositories.

However, you can also explicitly configure the context to be shared, like this:

container.Register(Component.For<DbContext>().LifeStyle.Singleton);

If you were to use another DI Container, the API would be different, but the concept is the same.

Bonus info: I don't know what the overall context is, but if this is to be used in a web application and DbContext is an Entity Framework or LINQ to SQL context, the correct lifetime configuration would instead be PerWebRequest as none of those context classes are thread-safe:

container.Register(Component.For<DbContext>().LifeStyle.PerWebRequest);