asmx web service returning xml instead of json

Finally figured it out.

The app code is correct as posted. The problem is with the configuration. The correct web.config is:

        <compilation debug="true" targetFramework="4.0" />
            <add name="ScriptHandlerFactory"
                 verb="*" path="*.asmx"
                 type="System.Web.Script.Services.ScriptHandlerFactory, System.Web.Extensions, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
                 resourceType="Unspecified" />

According to the docs, registering the handler should be unnecessary from .NET 4 upwards as it has been moved to the machine.config. For whatever reason, this isn't working for me. But adding the registration to the web.config for my app resolved the problem.

A lot of the articles on this problem instruct to add the handler to the <system.web> section. This does NOT work and causes a whole load of other problems. I tried adding the handler to both sections and this generates a set of other migration errors which completely misdirected my troubleshooting.

In case it helps anyone else, if I had ther same problem again, here is the checklist I would review:

  1. Did you specify type: "POST" in the ajax request?
  2. Did you specify contentType: "application/json; charset=utf-8" in the ajax request?
  3. Did you specify dataType: "json"in the ajax request?
  4. Does your .asmx web service include the [ScriptService] attribute?
  5. Does your web method include the [ScriptMethod(ResponseFormat = ResponseFormat.Json)] attribute? (My code works even without this attribute, but a lot of articles say that it is required)
  6. Have you added the ScriptHandlerFactory to the web.config file in <system.webServer><handlers>?
  7. Have you removed all handlers from the the web.config file in in <system.web><httpHandlers>?

Hope this helps anyone with the same problem. and thanks to posters for suggestions.