WCF MTOM/XOP Client Deserialization Error

Question

This is one of those SO 'questions' that i've already answered, but am posting b/c there seems to be next to zero information out there based on a week of Googling.

TL;DR: WCF MTOM encoded BasicHttpBinding client to external/3rd part, non-.NET web service chokes on XOP processing of MTOM response - basically MTOM encoder seems to be expecting a base64 payload in binary element, but runs in to the... directive and fails deserializing SOAP/XML to runtime object, thus throwing error in this question's Title.

Error: End element 'MyBinaryData' from namespace ' http://mynamespace ' expected. Found element 'xop:Include' from namespace ' http://www.w3.org/2004/08/xop/ '

As previously noted, there's not much out there on this topic, i'm guessing b/c MS wrote most of their WCF documentation based on service development, and not so much client (although there is some, to be fair).

I'm not going to go in to the nitty-gritty initial set up b/c i'm about to answer my own question, but i'll preface the answer by saying that this was much more akin to a default configuration of WCF MTOM than not.

Also, i know WCF is old, boring, and no longer actively developed by MS, but it is still supported and there are plenty of uses for it. I actually i didn't have much of a choice and had to find a way to make this work. This is why i'm sharing my findings with anyone else that has to deal with this kind of headache.

Answer 1

TL;DR: check http headers to see if service response is "Transfer-Encoding: chunked" (streamed) to you and if so, use transferMode="StreamedResponse" in your binding configuration.

So after Googling for days with no help, i spun up Fiddler for http traffic capture - this requires your WCF basic http binding configuration to proxy in to Fiddler ( http://localhost:8888 by default, i think) and depending on where your target service resides you may or may not need to configure Fiddler's Gateway settings (corporate proxy, etc.).

This allowed me to see the raw text being sent between my client to/from their service; all payloads were coming in just fine, which meant, in my case, that the MTOM/XOP response from the service was being completely transmitted and that the WCF runtime was not interpreting the response correctly. Another critical thing i saw was that the Transfer-Encoding http header was "chunked" and there was no Content-Length header... this meant that the service was streaming the response, as opposed to a buffered response. Now a little side note: MS's WCF MTOM documentation has a call-out saying that you should always use "Buffered" as your transferMode in your binding configuration... but failed to mention that was really only applicable in Services, not necessarily clients!

So naturally, i simply went in to my config file, found system.serviceModel >> bindings >> basicHttpBinding collection, found my specific binding configuration and set transferMode="StreamedResponse" (because the 3rd party service was streaming my response back to my client).