Controlling the WCF XmlSerializer

Question

I have some REST web services implemented in WCF. I wish to make these services return "Bad Request" when the xml contains invalid elements.

The xml serialization is being handled by XmlSerializer. By default XmlSerializer ignores unknown elements. I know it is possible to hook XmlSerializer.UnknownElement and throw an exception from this handler, but because this is in WCF I have no control over serialization. Any ideas how I might implement this behavior.

Answer 1

Maybe you can return your own type implementing IXmlSerializable and thorw the exception you want in the ReadXml and WriteXml methods...

Answer 2

This is from vague memory as I don't have all the code to hand, but you can create a custom Message (inherit for the class "Message") type to return in your REST services and override certain methods to create custom responses.

    protected override void OnWriteMessage(XmlDictionaryWriter writer)
    {
        ...
    }

    protected override void OnWriteStartEnvelope(XmlDictionaryWriter writer)
    {
        ...
    }

    protected override void OnWriteStartBody(XmlDictionaryWriter writer)
    {
        ...
    }

    protected override void OnWriteBodyContents(XmlDictionaryWriter writer)
    {
        ...
    }

Not a complete answer, but might push you down the right path.

Answer 3

"I know it is possible to hook XmlSerializer.UnknownElement and throw an exception from this handler, but because this is in WCF I have no control over serialization"

Its actually possible to do this...

In a WCF project that I worked on, we did something similar using the IDispatchMessageFormatter interface.

More information can be found here http://nayyeri.net/blog/use-idispatchmessageformatter-and-iclientmessageformatter-to-customize-messages-in-wcf/

It allows you peak at the message headers, control serialization/deserialization, return status codes etc.