Hey, I believe that everyone of us have ever dealt with versioning. Versioning inside of WCF doesn’t exist if so to say, but it is very easy to keep all your stuff running until you actually have to validate schema of the messages.

In recent time me with my team had to make our Phone client of version 1.0 and 1.1 work with either version of server 1.0 or 1.1. In other words we had 4 cases. Two of them are obviously non-problematic and two are not that clear. Accordingly to MSDN there is no direct support for versioning in WCF, except of that each data elements are optional by default. This means that if you haven’t specified opposite all of your DataMembers are going to be null if they are missed in incoming XML.

So having this in mind I expected that all of our versioning scenarios will go smoothly, but this did not happen. We were getting ugly exception looking like this: “The resource MultipleIdDefinition was not found”. Honestly it was very surprisingly since google search did not give me any good answer for this.

So here are steps that I took to figure out where the problem is:

• Made the way up to the deserialization code.
• Debugged to the point where I can see actual payload.
• For same data request I copied payloads as is from two server versions.
• Did text comparison and found what I was looking for.

Added properties could not brake serialization, so I found other differences, which were additional z:Id=”i7” right after one of the data contracts. Some of you might know that this is cause of IsReference=true of the DataContract. So I did research if this really has been changed between version 1.0 and 1.1 and found out that it was.

So, long story short, if you having some issues with serialization inside of WCF when dealing with versioning think less about new or even deleted elements of you contract, but think more what can cause mismatch in messages. And dong forget to see source control logs.