how let an asmx record cost json

i combined an asmx record formula behind file. it's operative fine, nonetheless outputting xml.

however, i need cost json. responseformat settlement doesn't seem work. code-behind is:

[system.web.script.services.scriptservice]
public category _default : system.web.services.webservice {
    [webmethod]
    [scriptmethod(usehttpget = true,responseformat = responseformat.json)]
    open string[] userdetails()
    {
        relapse new string[] { "abc", "def" };
    }
}