<responce>
  <post_id>1</post_id>
</responce>

<error>
   <error_code>1</error_code>
   <error_description>Произошла ошибка</error_description>
</error>

    [ServiceContract]
    [XmlSerializerFormat]
    public interface IFlickrApi
    {
        [OperationContract]
        [WebGet(BodyStyle = WebMessageBodyStyle.Bare, ResponseFormat = WebMessageFormat.Xml,
        UriTemplate = "method/wall.post.xml?owner_id={owner_id}&message={message}&access_token={access_token}")]
        VK_Response WallPost(string owner_id, string message, string access_token);
    }

    [XmlRoot("error")]
    public class VK_error
    {
        [XmlElement("error_code")]
        public string error_code { get; set; }

        [XmlElement("error_msg")]
        public string error_msg { get; set; }
    }

    [XmlRoot("responce")]
    public class VK_Response//: VK_error
    {
        [XmlElement("post_id")]
        public string post_id { get; set; }
    }

    [XmlRoot("*")]
    public class VK_Any
    {
        [XmlElement("error_code")]
        public string error_code { get; set; }

        [XmlElement("error_msg")]
        public string error_msg { get; set; }

        [XmlElement("post_id")]
        public string post_id { get; set; }
    }