Problem while create a new asmx page

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ASP.NET

webservice

Hi all

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";
    }
    
}

Please help me to solve the problem

Thanks in advance