gistfile1.cs

public class BlogService
{
    private Repository _repository;

    public BlogService()
    {
        _repository = new Repository();
    }

    public void ProcessComment(Comment comment)
    {
        _repository.Save(comment);

        // Send notification to author and do other stuff here
    }
}

gistfile2.cs

public interface IRepository
{
    void Save(Comment comment);
}

public class BlogService
{
    private IRepository _repository;

    public BlogService(IRepository repository)
    {
        _repository = repository;
    }

    public void ProcessComment(Comment comment)
    {
        _repository().Save(comment);

        // Send notification to author and do other stuff here
    }
}

gistfile3.cs

public class Repository : IRepository
{
    public void Save(Comment comment)
    {
        // Save the comment
    }
}