运行webService错误：使用 XmlInclude 或 SoapInclude 属性静态

System.InvalidOperationException: 生成 XML 文档时出错。 ---> System.InvalidOperationException: 不应是类型 Space.Entity.SpaceDepartment。使用 XmlInclude 或 SoapInclude 属性静态指定非已知的类型。
   在 System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive(String name,String ns,Object o,Boolean xsiType)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write2_Object(String n,Boolean isNullable,Boolean needType)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write8_ArrayOfAnyType(Object o)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer.Serialize(Object objectToSerialize,XmlSerializationWriter writer)
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter,XmlSerializerNamespaces namespaces,String encodingStyle,String id)
   --- 内部异常堆栈跟踪的结尾 ---
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter,String id)
   在 System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter,Object o)
   在 System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response,Stream outputStream,Object returnValue)
   在 System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues,Stream outputStream)
   在 System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   在 System.Web.Services.Protocols.WebServiceHandler.Invoke()

注：其中webservices方法的返回值为：IList.

解决方法参考：http://chocoboboy.spaces.live.com/blog/cns!1983ad9c55f2f5c7!323.entry?wa=wsignin1.0