I have been working with a WCF service that has implemented IExtensibleDataObject, and when I pass objects into my client-side script, the object has an "ExtensionData" field in it. That's fine for passing the data in, but then when I try to pass that object back from the JS to a method in the service, it dies there. The error I get is "No parameterless constructor defined for type of 'System.Runtime.Serialization.ExtensionDataObject'."
We tried the class without implementing IExtensionDataObject but that we want to implement it for versioning reasons, and we tried decorating the ExtensionData property with [DataMember], but that just caused the service to fail altogether.
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