As some of you may know, correcting Joel is one of my hobbies. But being able to add to a Tanguy’s tip takes it to a completely new level. To recall: our challenge is to find all parameters including their types specific to a plugin message.
Instead of writing some temporary code only to delete it later, how about finding microsoft.xrm.sdk assembly in your project references and use Visual Studio Object Browser:
Note: inherited properties like RequestId, RequestName, etc, are common for all messages and can be found at the OrganizationRequest object level.