How to let an ASMX file output JSON

I created an ASMX file with a code behind file. It's working fine, but it is outputting XML.

However, I need it to output JSON. The ResponseFormat configuration doesn't seem to work. My code-behind is:

public class _default : System.Web.Services.WebService {
    [ScriptMethod(UseHttpGet = true,ResponseFormat = ResponseFormat.Json)]
    public string[] UserDetails()
        return new string[] { "abc", "def" };


From WebService returns XML even when ResponseFormat set to JSON:

Make sure that the request is a POST request, not a GET. Scott Guthrie has a post explaining why.

Though it's written specifically for jQuery, this may also be useful to you:
Using jQuery to Consume ASP.NET JSON Web Services