var posts = _api.Wall.Get(new WallGetParams { Domain = "some domain", Count = 1 });
foreach (var item in posts.WallPosts)
{
for (int i = 0; i < item.Attachments.Count; i++)
{
Photo items = (Photo)item.Attachments[i].Instance;
List<Photo> post = new List<Photo>();
post.AddRange(items); // тут ошибку выбивает
}
}