WCF: Element has already been exported error

Question

I've acquired more information since the original post and have been able to recreate the problem in a very simple File > New Project > WCF Service Application solution. As a result I'm heavily editing the original content of this post to get rid of some of the superfluous information and simplify the examples:

We have a message contract defined as follows.

[MessageContract( WrapperName = "SingleTypeResponse", WrapperNamespace = "urn:WcfService1" )]
public class SingleTypeResponse<T>
{
    [MessageBodyMember( Name = "ReturnValue" )]
    public T ReturnValue { get; set; }
}

The service interface has the following:

[OperationContract]
SingleTypeResponse<string> GetStringData();

[OperationContract]
SingleTypeResponse<int> GetIntData();

When I run the project and navigate to the .svc file I get the following:

An exception was thrown in a call to a WSDL export extension: System.ServiceModel.Description.DataContractSerializerOperationBehavior contract: http://tempuri.org/:IService1 ----> System.InvalidOperationException: The WcfService1.IService1.GetIntData operation references a message element [urn:WcfService1:SingleTypeResponse] that has already been exported from the WcfService1.IService1.GetStringData operation. You can change the name of one of the operations by changing the method name or using the Name property of OperationContractAttribute. Alternatively, you can control the element name in greater detail using the MessageContract programming model.

If I comment out GetIntData on the interface and test the service using the WCF Test Client, with the WrapperName property set, I get the following response XML:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Header />
  <s:Body>
    <SingleTypeResponse xmlns="urn:WcfService1">
      <ReturnValue xmlns="http://tempuri.org/">foo</ReturnValue>
    </SingleTypeResponse>
  </s:Body>
</s:Envelope>

I'm guessing that element is the source of the problem, it sees two versions of the same element, one with a string and one with an int.

I then uncommented the GetIntData and removed the WrapperName property from the MessageContract:

[MessageContract( WrapperNamespace = "urn:WcfService1" )]
public class SingleTypeResponse<T>
{
    [MessageBodyMember( Name = "ReturnValue" )]
    public T ReturnValue { get; set; }
}

I get the same error message with the exception that the message element it's complaining about is the ReturnValue property of the contract rather than the message contract name.

Once again commenting out GetIntData and testing with WCF Test Client I get:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
  <s:Header />
  <s:Body>
    <SingleTypeResponseOf_String xmlns="urn:WcfService1">
      <ReturnValue xmlns="http://tempuri.org/">foo</ReturnValue>
    </SingleTypeResponseOf_String>
  </s:Body>
</s:Envelope>

So I'm able to get it to create a unique name for the wrapper, but the fact that SingleTypeResponseOf_String and SingleTypeResponseOf_Int32 both have a ReturnValue property continues to cause it to blow its brains out.

I started to write an epic response but thought better of it because I think I misdiagnosed - it looks like you are namespacing your message contracts. There's no reason that WCF won't allow two operations to return the same generic response object. Do you have any more info on what's happening? — Kirk Broadhurst
Our best guess at this point is that the WSDL contains two conflicting definitions for SingleTypeResponse, one for the string version and one for the int version. When WCF Storm sees that it can't figure out how to generate the proxy. Our concern of course is that if WCF Storm can't figure out how to generate the proxy then other tools that try to use a service reference won't be able to figure it out either. svcutil gives the same error when run from the command-line. I've been fighting trying to get the WSDL from the servcie so we can look at it. We use net.tcp endpoints. — Craig W.

Kirk Broadhurst Kirk Broadhurst · Accepted Answer · 2011-09-22T00:23:28

I've never thought about it before but it makes sense that generic types need to be defined as concrete types for a WSDL. Coming from that perspective, it appears that yes - you are defining two 'types' of SingleTypeResponse in your service. WSDL does not allow two element definitions of the same name & namespace to coexist (for obvious reasons - they need to be uniquely identifiable).

There are a couple of potential conflicts here. I think you've identified the first one - an element named SingleTypeResponse within the urn:WcfService1 namespace. When you turn off the naming of the message contract (allowing it to be named by the serializer) you see the below:

<!-- Setting wrapper name & wrapper namespace -->
<wsdl:message name="SingleTypeResponseOf_String">
  <wsdl:part name="parameters" element="q1:SingleTypeResponse" 
             xmlns:q1="urn:WcfService1" /> 
</wsdl:message>

<!-- Setting wrapper namespace only -->
<wsdl:message name="SingleTypeResponseOf_String">
  <wsdl:part name="parameters" element="q1:SingleTypeResponseOf_String" 
             xmlns:q1="urn:WcfService1" /> 
</wsdl:message>

This should avoid the first conflict, because running your two operations means that you'll be able to have two types of element (SingleTypeResponseOf_String and SingleTypeResponseOf_Int).

I believe your second conflict comes from the MessageBodyMember attribute. Because both the operations define messages in the same namespace, and both return types contain an element ReturnValue, you will get a conflict where the urn:ReturnValue element is defined twice, once as an int and once as a string.

To demonstrate see the following, with the GetIntData operation comment out, see the ReturnValue element defined:

<!-- from the XSD http://localhost/Service1.svc?xsd=xsd0 -->
<xs:element name="SingleTypeResponseOf_String">
  <xs:complexType>
    <xs:sequence>
      <xs:element minOccurs="0" ref="q1:ReturnValue" 
                  xmlns:q1="http://tempuri.org/" /> 
    </xs:sequence>
  </xs:complexType>
</xs:element>

<!-- from the XSD http://localhost/Service1.svc?xsd=xsd2 -->
<xs:schema elementFormDefault="qualified" 
           targetNamespace="http://tempuri.org/" 
           xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           xmlns:tns="http://tempuri.org/">
  <xs:element name="ReturnValue" 
              nillable="true" 
              type="xs:string" /> 
</xs:schema>

How can you allow the MessageContract to rename the ReturnValue property? I don't think you can with DataContractSerializer.

What's the solution? Well, I don't think there is a solution using [MessageBodyMember]. If you use [DataMember] it will work fine. You mentioned before that you're using net.tcp, so I assume your .NET to .NET. Do you need to exercise that level of control of the SOAP envelopes?

Normally I use DataContract as much as possible, and only venture into MessageContract when necessary - interfacing with older SOAP style platforms.

WCF: Element has already been exported error

1 Answers