User Voice DDD - domain implementation

UserVoiceDddDomain.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace UserVoiceFun.Domain
{
    public abstract class Entity
    {
        public Guid Id { get; protected set; } = Guid.NewGuid();
    }

    public class User : Entity // Aggregate Root
    {
        public string Key => $"{Email}:{MarketplaceUrl}";

        public string Name { get; }

        public string Email { get; }

        public string MarketplaceName { get; }

        public Uri MarketplaceUrl { get; }

        internal User(string name, string email, string marketplaceName, Uri marketplaceUrl)
        {
            Name = name;
            Email = email;
            MarketplaceName = marketplaceName;
            MarketplaceUrl = marketplaceUrl;
        }

        public Suggestion MakeSuggestion(string text)
        {
            return new Suggestion(Id, text);
        }
    }

    public class Suggestion : Entity // Aggregate Root
    {
        public string Text { get; /* a suggestion cannot be altered */ }

        public Guid ByUserId { get; }

        public DateTime SuggestedAt { get; }

        public ICollection<Comment> Comments { get; } = new List<Comment>();

        public ICollection<Vote> Votes { get; } = new List<Vote>();

        internal Suggestion(Guid byUserId, string text)
        {
            ByUserId = byUserId;
            Text = text;
            SuggestedAt = DateTime.UtcNow;
        }

        public Comment AddComment(Guid byUserId, string text)
        {
            var comment = new Comment(byUserId, text);
            Comments.Add(comment);
            return comment;
        }

        public void RemoveComment(Comment comment, Guid userIdRemovingComment)
        {
            if (comment.ByUserId != userIdRemovingComment)
                throw new CannotRemoveCommentFromAnotherUserException();

            Comments.Remove(comment);
        }

        public void Unvote(Guid byUserId)
        {
            var vote = Votes.SingleOrDefault(v => v.ByUserId == byUserId);

            if (vote != null)
                Votes.Remove(vote);
        }

        public Vote Vote(Guid byUserId)
        {
            if (Votes.Any(v => v.ByUserId == byUserId))
                throw new CannotVoteTwiceOnSameSuggestionException();

            var vote = new Vote(byUserId);
            Votes.Add(vote);
            return vote;
        }
    }

    public class Comment : Entity
    {
        public string Text { get; /* a comment cannot be changed */ }

        public Guid ByUserId { get; }

        public DateTime CommentedAt { get; }

        internal Comment(Guid byUserId, string text)
        {
            ByUserId = byUserId;
            Text = text;
            CommentedAt = DateTime.UtcNow;
        }
    }

    public class Vote : Entity
    {
        public Guid ByUserId { get; }

        public DateTime VotedAt { get; }

        internal Vote(Guid byUserId)
        {
            ByUserId = byUserId;
            VotedAt = DateTime.UtcNow;
        }
    }

    public interface IUsersStore
    {
        Task AddUserAsync(User user);

        Task<User> GetUserAsync(Guid id);
    }

    public interface ISuggestionsStore
    {
        Task AddSuggestionAsync(Suggestion suggestion);

        Task<Suggestion> GetSuggestionAsync(Guid id);

        // For when comments and votes are added/removed to/from a suggestion.
        Task UpdateSuggestionAsync(Suggestion suggestion);
    }

    public class UserVoiceService
    {
        private readonly IUsersStore usersStore;

        public UserVoiceService(IUsersStore usersStore)
        {
            this.usersStore = usersStore;
        }

        public async Task<User> RegisterUserAsync(string name, string email, string marketplaceName, Uri marketplaceUrl)
        {
            var user = new User(name, email, marketplaceName, marketplaceUrl);
            await usersStore.AddUserAsync(user);
            return user;
        }
    }

    public class CannotVoteTwiceOnSameSuggestionException : Exception { }

    public class CannotRemoveCommentFromAnotherUserException : Exception { }
}

A request for reviewing this model was posted to Code Review at Stack Exchange. Since then, this gist has been updated to reflect the accepted suggestions.