C# enumeration property null vs. 0

Question

I'm using IIS/asmx to support a Flash client. Some of my service layer data transfer objects have properties that are enumeration values. There are cases where these properties should be null.

When an object with a null value for such an enumeration property is rendered to soap, I receive this error:

System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: Instance validation error: '0' is not a valid value for NameSpaceX.Model.NodeType.
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write1_NodeType(NodeType v)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write4_PackageDTO(String n, String ns, PackageDTO o, Boolean isNullable, Boolean needType)
   at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write15_ArrayOfPackageDTO(Object o)
   at Microsoft.Xml.Serialization.GeneratedAssembly.ListOfPackageDTOSerializer1.Serialize(Object objectToSerialize, XmlSerializationWriter writer)
   at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   --- End of inner exception stack trace ---
   at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o, XmlSerializerNamespaces namespaces)
   at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o)
   at System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response, Stream outputStream, Object returnValue)
   at System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
   at System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   at System.Web.Services.Protocols.WebServiceHandler.Invoke()

Is there a better solution then to create an enumeration member with value 0 (e.g. [0, "null"]? I suspect that I'm lacking basic knowledge here.

Thanks.

Answer 1

My first inclination was to point you to the Nullable<T> type. However, after looking around for documentation on Nullable<T> and SOAP, it appears that it might not be supported. You might want to explore this further.

That said, it is very common for Enums to declare a NotSet = 0 member.