如何在windows phone 8中将friendlist绑定为longlistselector

我在windows phone中制作一个应用程序聊天，我有一个类名GetFriendResponse

    using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace RAMACHAT.Model
{
    public class FriendInfo
    {
        public bool success { get; set; }
        public int statusCode { get; set; }
        public string message { get; set; }
        public _user data { get; set; }
    }
    public class GetFriendResponse
    {
        public bool success { get; set; }
        public int statusCode { get; set; }
        public string message { get; set; }
        public string _id { get; set; }
        public List<_user> data { get; set; }
    }

和class _user作为

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace RAMACHAT.Model
{
    public class _user
    {
        public string _id { get; set; }
        public string username { get; set; }
        public string avatar { get; set; }
        public bool isOnline { get; set; }
        public bool isFollow { get; set; }
    }
}

    }

现在我想让friendslist作为longlistselector当服务器响应用户friendslist

 string result = await App.client.getAllFriends();
            string resultHistory = await App.client.getChatHistory();
            resultObject = JsonConvert.DeserializeObject<GetFriendResponse>(result);
            var resultHistoryObject = JsonConvert.DeserializeObject<HistoryResponse>(resultHistory);
            List<AlphaKeyGroup.AlphaKeyGroup1<_user>> DataSource = AlphaKeyGroup.AlphaKeyGroup1<_user>.CreateGroups(resultObject.data,
                           System.Threading.Thread.CurrentThread.CurrentUICulture,
                           (_user s) => { return s.username; }, true);
            friendList.ItemsSource = DataSource; 

但它不工作，谁有一个想法来修复它，谢谢这么多!