WCF REST服务没有'；I don’我想我有一个循环推荐信，所以我什么都不回

我正在使用实体框架代码优先数据层开发WCF REST服务，并且我有一个导航属性。

用户类别：

[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; }
}

服务合同方法：

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;
}

此方法不返回任何内容。如果我评论这行FromWhomIsFriend = f.FromWhomIsFriend，它会起作用。

FromWhomIsFriend是我是他的朋友的用户的导航属性。为了表示用户关系，我有一个表：

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

如果我询问用户1的朋友，我会得到用户2，它的FromWhomIsFriend指向用户1。并且用户1 Friends导航属性指向用户2，并继续。

你知道我为什么什么都不回吗

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