Question

I have a webservice that always returns a ReturnObject that is a class I have written like this:

public class ReturnObject
{
    public Status Status { get; set; }
    public Object Data { get; set; }
}

When I return a single object like an int or string etc in Data it is OK but when I want to return a list of typed objects the client that running the webservice as a WebReference getting this exception:

System.Web.Services.Protocols.SoapException: Server was unable to process request.
—> System.InvalidOperationException: There was an error generating the XML document
—> System.InvalidOperationException:
Typen
System.Collections.Generic.List`1[[Transticket.Domain.Models.ApsisMailingList,
Transticket.Domain, Version=1.0.0.0, Culture=neutral,
PublicKeyToken=null]] kan inte användas i den här kontexten. vid
System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive(String
name, String ns, Object o, Boolean xsiType) vid
Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write1_Object(String
n, String ns, Object o, Boolean isNullable, Boolean needType) vid
Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write3_ReturnObject(String
n, String ns, ReturnObject o, Boolean isNullable, Boolean needType)
vid
Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write6_GetListsResponse(Object[]
p) vid
Microsoft.Xml.Serialization.GeneratedAssembly.ArrayOfObjectSerializer5.Serialize(Object
objectToSerialize, XmlSerializationWriter writer) vid
System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter,
Object o, XmlSerializerNamespaces namespaces, String encodingStyle,
String id) — Slut på stackspårning för interna undantag —
vid System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter
xmlWriter, Object o, XmlSerializerNamespaces namespaces, String
encodingStyle, String id) vid
System.Web.Services.Protocols.SoapServerProtocol.WriteReturns(Object[]
returnValues, Stream outputStream) vid
System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[]
returnValues) vid
System.Web.Services.Protocols.WebServiceHandler.Invoke() — Slut
på stackspårning för interna undantag —

This exception is not in the webservice, it’s in the client call to the webservice after returning the data.

If I do another ReturnObject there and I specify a list of typed objects as data, it works well.

Someone have an idea how I generalize this? I have always thought that List is a type of Object but looks like a .NET bug, or I have missed something?

Answer 1

XmlSerializer does not include type information in the output (contrast with BinaryFormatter); as such, there is no robust way of deserializing object, as the xml could be (quite literally) anything.

The same is true of most contract-based serializers (DataContractSerializer, protobuf-net, etc).

Basically: don’t do that. object is not a useful thing to use on a web-service, and that cannot work.