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

悬赏园豆：50 [待解决问题]

[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) };
        }
    }

调用方法(GetMsg/SendMsg)时报错:

System.MethodAccessException: 方法“System.ServiceModel.Description.TypeLoader.GetKnownTypes(System.Object[], System.Reflection.ICustomAttributeProvider)”访问方法“WCFSLClient.ServiceReference1.Helper.GetKnownTypes(System.Reflection.ICustomAttributeProvider)”的尝试失败。

Silverlight的WCF代理是手写代码不是自动生成的. 在相关服务契约上加了ServiceKnownType特性

WCF Silverlight WCF数据契约 ServiceKnownType KnownType

问题补充：

为什么没人理呀..