Question

I am creating an asmx web service.
In visual studio 2012 on the project name I right clicked and added an web service name UserDetails.asmx

Now I am trying to use the web service in the js file. like this

$.ajax({
    type: "POST",
    url: "UserDetails.asmx/HelloWorld",
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function (data) {
       alert('success');
    },
    error: function () {
        alert("Error");
    }
});

Its showing error that POST http: //192.168.9.185/GPS/UserDetails.asmx/HelloWorld 500 (Internal Server Error)

Is there any fault in my code or I am missing something so that its showing error.
My Asmx Page

<%@ WebService Language="C#" CodeBehind="~/App_Code/UserDetails.cs" Class="UserDetails" %>

My Code Behind

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;

/// <summary>
/// Summary description for UserDetails
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
// [System.Web.Script.Services.ScriptService]
public class UserDetails : System.Web.Services.WebService {

public UserDetails () {

    //Uncomment the following line if using designed components 
    //InitializeComponent(); 
}

[WebMethod]
public string HelloWorld() {
    return "Hello World";
}

}

Answer 1

Edit

Short Answer: just uncomment [System.Web.Script.Services.ScriptService]

Digging deep in the error message by using

error: function (xhr,status, error) {
        alert("Error");
       }

and inspecting xhr revealed that the actual error was something else (Only Web services with a [ScriptService] attribute on the class definition can be called from script).

I used [WebMethod] in my aspx code behind and whenever I received such message, I found out that there was an error in the data I am passing. So try adding data:"{}", in your $.ajax()

as described in this post

When making a read-only request, the empty data parameter is the key. For reasons unknown, jQuery does not properly set the specified content-type when there is no data included.