First WCF Request, client always throws System.Xml.XmlException: There are multiple root elements

Question

Every time my WCF Host application pool starts, the client making the first WCF call to it always throws "System.Xml.XmlException: There are multiple root elements" All subsequent calls work perfectly.

This exception happens on the consumer/client side of the WCF request. I've tested this for both a full WCF client and a Silverlight client. It's using basicHttpBinding, no security, and aspnetCompatabilityMode = true

It wouldn't be a big deal if the app pool stayed alive, but with the lack of activity, it shuts down and the error happens again when it starts back up.

I should also mention that the app pool sometimes starts from a non-WCF request to another page. But still the first time WCF is called, it still throws this exception on the client side.

Has anyone seen this? I can provide more details if necessary.

Thanks

Answer 1

It sounds to me like a serialization issue. I would investigate how you build your DataContracts -- you are using DataContracts and not the XML serialization attributes, right?

Edit: Based on our comments, I'm going to give a recommendation for your refactoring:

[DataContract] 
public class ImageEffectExcludeParamRequest
{ 
    [DataMember] 
    public string ImageID { get; set; } 

    [DataMember] 
    public int EffectID { get; set; } 

    [DataMember]
    public ResponseInfo AdditionalInfo { get; set; }
}


[DataContract] 
public class ResponseInfo 
{ 
    [DataMember] 
    public Enums.ServiceResponse.Status Status { get; set; } 

    [DataMember] 
    public string Message { get; set; } 
}

Using composition rather than inheritance should solve your problem.

Answer 2

Okay, after researching these options, I figured out what caused the issue. Ultimately, inheritance and having both attributes, serializable and DataContract, in the data being exchanged did not make a difference with deserializing the response.

The real meat of the issue was in my configuration. Earlier I was playing with Streaming messages. I left my host transferMode set to Streaming and my client was set to Buffered. In silverlight, that's my only option. So the serialization problem happened because the message was being chunked. I noticed this after tracing a few calls.

So easy peasy fix. Switch transferMode to Buffered. I'm going to setup a separate endpoint for streaming and play with that another time. I don't need to stream the CRUD services.

Thanks for everyone's input.

-Nathan