How to avoid anemic domain models, or when to move methods from the entities into services

Question

I have a common scenario that I am looking for some guidance from people more experienced with DDD and Domain Modeling in general.

Say I start out building a blog engine

Answer 1

Without using domain events, you can use the Double Dispatch pattern and put the AddComment logic inside a Domain Service.

This is how it would look like:

public class Article
{
    public void AddComment(Comment comment, IAddCommentProcessor commentProcessor)
    {
        commentProcessor.AddComment(this, comment);
    }
}

public interface IAddCommentProcessor
{
    void AddComment(Article article, Comment comment);
}

public class AddCommentAndEmailProcessor : IAddCommentProcessor
{
    private readonly _emailService;
    public AddCommentAndEmailProcessor(EmailService emailService)
    {
        _emailService = emailService;
    }

    public void AddComment(Article article, Comment comment)
    {
        // Add Comment

        // Email
        _emailService.SendEmail(App.Config.AdminEmail, "New comment posted!");
    }
}

public class ArticleController
{
    private readonly IRepository _repository;
    private readonly IArticleService _articleService;

    public ArticleController(IRepository repository, IArticleService articleService)
    {
        _repository = repository;
        _articleService = articleService;
    }

    public void AddComment(int articleId, Comment comment)
    {
        var article = _repository.Get
(articleId);
        article.AddComment(comment, new AddCommentAndEmailProcessor(ServiceLocator.GetEmailService())); // Or you can use DI to get the Email Service, or any other means you'd prefer
        _repository.Save(article);
        return RedirectToAction("Index");
    }
}

If you prefer, you can keep the Adding Comment logic on the AddComment of Article, and instead make the Domain Service into something like ICommentAddedProcessor with a CommentAdded() method and have AddComment on Article take a Comment and a ICommentAddedProcessor.