Почему не работает получение ссылки на фото из xml ВКонтакте? - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Drawing.Text;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Xml.Linq;
 
namespace WindowsFormsApplication16
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
 
            dataGridView1.DataSource = GetVkpostslist();
 
        }
 
        public class Vkpost
        {
            public int ID { get; set; }
            public int from_id { get; set; }
            public int owner_id { get; set; }
            public DateTime date { get; set; }
            public string post_type { get; set; }
            public string text { get; set; }
            public bool is_pinned { get; set; }
            public int comments { get; set; }
            public int likes { get; set; }
            public int reposts { get; set; }
            public attachmentslist attachmentslist { get; set; }
        }
 
        public class attachmentslist
        {
           public List<imginfo> img = new List<imginfo>();
        }
 
        public class imginfo
        {
            public string photo_75 { get; set; }
            public string photo_130 { get; set; }
            public string photo_604 { get; set; }
            public string idphoto { get; set; }
 
        }
 
        public List<Vkpost> GetVkpostslist()
        {
            var webRequest =
                HttpWebRequest.Create("http://api.vk.com/method/wall.get.xml?v=5.45&filter=all&domain=sci&count=5");
            var webResponse = webRequest.GetResponse();
            var stream = webResponse.GetResponseStream();
            var xmlDoc = XDocument.Load(stream);
           var Vkpost =
                from post in xmlDoc.Descendants("post")
                select new Vkpost
                {
                    ID = Convert.ToInt32(post.Element("id").Value),
                    text = post.Element("text").Value,
                    likes = Convert.ToInt32(post.Element("likes").Value),
                    attachmentslist = new attachmentslist()
                    {
                        img = new List<imginfo>(from im in post.Descendants("photo")
                            select new imginfo()
                            {
                                photo_604 = im.Element("photo_604").Value,
                                photo_130 = im.Element("photo_130").Value,
                                idphoto = im.Element("id").Value,
                                photo_75 =im.Element("photo_75").Value
 
                            })
                    }
                };
            return Vkpost.ToList();
        }
 
    }
}