WCF REST service doesn't return anything because I have a circular reference (I think)

Question

I'm developing a WCF REST service using Entity Framework Code First data layer and I have a navigation property.

User class:

[DataContract]
public class User
{
    [DataMember]
    public int UserId { get; set; }

    [DataMember]
    public string Name { get; set; }

    [DataMember]
    public int Age { get; set; }

    [DataMember]
    public string City { get; set; }

    [DataMember]
    public string Country { get; set; }

    [DataMember]
    public string Email { get; set; }

    [DataMember]
    public string InterestIn { get; set; }

    [DataMember]
    public virtual ICollection<User> Friends { get; set; }

    [DataMember]
    public virtual ICollection<User> FromWhomIsFriend { get; set; }
}

ServiceContract method:

public List<User> GetUserFriends(string user_id)
{
    int userId;
    OutgoingWebResponseContext ctx =
        WebOperationContext.Current.OutgoingResponse;

    if ((user_id == null) ||
        (!Int32.TryParse(user_id, out userId)) ||
        (userId < 1))
    {
        ctx.StatusCode = System.Net.HttpStatusCode.BadRequest;
        ctx.StatusDescription = "user_id parameter is not valid";
        throw new ArgumentException("GetUserFriends: user_id parameter is not valid", "user_id");
    }

    List<User> friends = null;

    try
    {
        using (var context = new AdnLineContext())
        {
            context.Configuration.ProxyCreationEnabled = false;
            context.Configuration.LazyLoadingEnabled = false;

            var users = from u in context.Users.Include("Friends")
                        where u.UserId == userId
                        select u;

            if ((users != null) &&
                (users.Count() > 0))
            {
                User user = users.First();
                //friends = user.Friends.ToList();

                friends = new List<User>();
                foreach (User f in user.Friends)
                {
                    User us = new User()
                    {
                        UserId = f.UserId,
                        Name = f.Name,
                        Age = f.Age,
                        City = f.City,
                        Country = f.Country,
                        Email = f.Email,
                        InterestIn = f.InterestIn,
                        Friends = f.Friends,
                        FromWhomIsFriend = f.FromWhomIsFriend
                    };
                    friends.Add(us);
                }
            }

            ctx.StatusCode = System.Net.HttpStatusCode.OK;
        }
    }
    catch (Exception ex)
    {
        ctx.StatusCode = System.Net.HttpStatusCode.InternalServerError;
        ctx.StatusDescription = ex.Message; 
        ctx.SuppressEntityBody = true;
    }

    return friends;
}

This method doesn't return anything. If I comment this line FromWhomIsFriend = f.FromWhomIsFriend it works.

FromWhomIsFriend is a navigation property to the user from whom I am his friend. To represent user relationship I have this table:

UserID   | FriendID
---------+----------
  3      |    1
---------+----------
  1      |    2

If I ask about friends from user 1, I get user 2, and its FromWhomIsFriend pointing to user 1. And user 1 Friends navigation property pointing to user 2, and continues.

Do you know how why I don't return anything?

Answer 1

You have to enable proxy creation in order to support the lazy loading. What you can do is to use Include in your query to load the navigation property .

        var users = from u in context.Users.Include(u=>u.Friends)
                    where u.UserId == userId
                    select u;

Or else the other solution is using separate object model as the WCF contracts(DTO). Then you can enable lazy loading and then copy all the required values form the EF entity object(proxy) to your new Data Transfer Object. You can use something like Automaper to map the object easily.