Say What?! That’s exactly what I said when I first saw this error. Come to find out the cause was nothing even close to what the message stated.
Here’s the deal: I use the “Publish” feature built into Visual Studio to deploy my projects to our Development server. This feature keeps track of what has changed between your last deployment and the one open in Visual Studio by writing information to an XML file. For the most part this works beautifully, and I’m thankful to the VS team for providing this useful feature.
However, I recently pushed changed out that somehow didn’t make it to the target location (a.k.a. the DLL’s that should have been updated weren’t). It just so happened that these DLL’s were part of the WCF Services my Silverlight project connects to. When Silverlight made the web service call the old DLL’s executed, behaving in a manner different than what my Silverlight project expected. The end result was the Web Service puking out this error.
Delete the files from your target prior to Publishing. You can do this by selecting this option in the Publish feature or by manually deleting them. This ensures that the most up-to-date files will be pushed to the target.