I faced a similar problem. The issue at hand is that the module that does the serialization/deserialization will look to the return types and parameters as cues for what it should serialize to / deserialize to , and it doesn't look deeper than that. I ended up doing a fairly hacky work-around to get the code in on time, but at some point I'm probably going to try and figure out a good solution.
A couple thoughts I had, neither of which are tried out. First, I'm aware that you can generate javascript types for nested types of the web service class (see http://ajax.asp.net/docs/mref/T_System_Web_Script_Services_GenerateScriptTypeAttribute.aspx) so that's an option, I suppose. Seems like not the best solution in our case, though, as what we're working with aren't logically nested types of the web service. I also considered writing a serializer module that can take the first param as a type argument so that, at the least, you could specify which concrete type you're sending upstream.instead of letting the module map only to the types in the webmethod params.
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