Silverlight 中 调用WCF数据契约中要传一个object 类型的属性.为什么用不了这个ServiceKnownType/KnownType,该如何处理

 [ServiceContract]    
 [ServiceKnownType("GetKnownTypes", typeof(Helper))]    
 public interface IMsgService    
 {        
   [OperationContract]        
   void SendMsg(Message message);  
      
   [OperationContract]        
   Message GetMsg();    
 }

 
    [DataContract]
    //[KnownType("GetKnownTypes")]
    public class Message
    {
        [DataMember]
        public string Content { get; set; }
        [DataMember]
        public object obj { get; set; }
       //static Type[] GetKnownTypes()
       //{
       //    return new[] { typeof(string) };
       //}
    }

  
    static class Helper
    {
        public static IEnumerable<Type> GetKnownTypes(ICustomAttributeProvider provider)
        {
            return new[] { typeof(string),..等(这里用时会动态取出要用的类型) };
        }
    }