When null is used as the event argument parameter, the string is serialized to "null" and sent back to the server. Because there is no way to distinguish between null and "null", you should avoid using null argument parameters with callbacks.

Exceptions thrown in ICallbackEventHandler.RaiseCallbackEvent(string) will not prevent ICallbackEventHandler.GetCallbackResult from being called and returning a result. To prevent ICallbackEventHandler.GetCallbackResult from returning a result, set a flag when an exception is thrown in ICallbackEventHandler.RaiseCallbackEvent(string) and return an empty string from ICallbackEventHandler.GetCallbackResult.