Feb 06, 2015 07:47 AM|PatriceSc|LINK
I recently checked the same issue. To me you just have to add a ScriptService attribute on your service class and use the proper application/json content type client side (in your $.ajax call) and you do have a json payload.
http://blog.prabir.me/posts/json-in-classical-web-services-asmx (it's a bit unclear IMO but the whole hand written conversion is not needed depending on your case, unfortunately he starts by that rather than by the simplest option, just use the simplest
option that works for you).