How to get JSON response from a 3.5 asmx web service

Question 1

I faced the same issue, and included the below code to get it work.

[WebMethod]
[ScriptMethod(UseHttpGet=true ,ResponseFormat = ResponseFormat.Json)]
public void HelloWorld()
{
    Context.Response.Clear();
    Context.Response.ContentType = "application/json";
    Context.Response.Write("Hello World");
    //return "Hello World";
}

Update:

To get a pure json format, you can use javascript serializer like below.

public class WebService1 : System.Web.Services.WebService
{
    [WebMethod]
    [ScriptMethod(UseHttpGet=true ,ResponseFormat = ResponseFormat.Json)]
    public void HelloWorld()
    {
        JavaScriptSerializer js = new JavaScriptSerializer();
        Context.Response.Clear();
        Context.Response.ContentType = "application/json";           
        HelloWorldData data = new HelloWorldData();
        data.Message = "HelloWorld";
        Context.Response.Write(js.Serialize(data));


    }
}

public class HelloWorldData
{
   public String Message;
}

However this works for complex types, but string does not show any difference.

Question 2

Dear future readers: The currently accepted answer is not the right way. It ties you to using the JavaScriptSerializer and you lose the ability to request xml (or indeed any serialization format which may come along in the future). The "right way" also involves less code!

If you decorate your service class with the [ScriptService] attribute - which you have - then ASP.NET 3.5+ should automatically serialise the response to JSON provided your Ajax call requests JSON. The suggestions to serialise to JSON manually are simply wrong, unless you wish to use a different serialiser such as Newtonsoft.

That you were seeing XML suggests one of the following:

You are not requesting JSON in your Ajax call - please see working example code below
Possibly some web.config entries are missing, according to an answer here (disclaimer: I don't have most of these in a production web.config; only start playing with these if nothing else works)

Here is a simple working example of a JSON enabled ASMX web service:

<%@ WebService Language="C#" Class="WebService" %>
using System;
using System.Collections.Generic;
using System.Web.Services;

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.Web.Script.Services.ScriptService]
public class WebService : System.Web.Services.WebService {
    [WebMethod]
    public MyClass Example()
    {
        return new MyClass();
    }

    public class MyClass
    {
        public string Message { get { return "Hi"; } }
        public int Number { get { return 123; } }
        public List<string> List { get { return new List<string> { "Item1", "Item2", "Item3" }; } }
    }
}

JavaScript to request it and process the response (we'll simply pop up a JS alert with the message from MyClass.Message) :

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Test</title>
    <script src="https://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.6.4.js" type="text/javascript"></script>  
</head>
<body>
    <script type="text/javascript">
        $.ajax({
            type: "POST",
            url: "WebService.asmx/Example",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            data: "{ }",
            error: function (XMLHttpRequest, textStatus, errorThrown) { alert(langError + " " + textStatus); },
            success: function (msg) {
                alert(msg.d.Message);
            }
        });
    </script>
</body>
</html>

Http request:

POST http://HOST.com/WebService.asmx/Example HTTP/1.1
Accept: application/json, text/javascript, */*; q=0.01
Content-Type: application/json; charset=utf-8
X-Requested-With: XMLHttpRequest
Referer: http://HOST.com/Test.aspx
Accept-Language: en-GB,en;q=0.5
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)
Connection: Keep-Alive
Content-Length: 3
Host: HOST.com

{ }

HTTP response:

HTTP/1.1 200 OK
Cache-Control: private, max-age=0
Content-Type: application/json; charset=utf-8
Server: Microsoft-IIS/8.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Tue, 20 Feb 2018 08:36:12 GMT
Content-Length: 98

{"d":{"__type":"WebService+MyClass","Message":"Hi","Number":123,"List":["Item1","Item2","Item3"]}}

Result:

"Hi" is displayed in a JS popup.

Question 3

Just a doubt. When are you not getting a JSON response? Because when you invoke the web service from the client (I am assuming a web browser, i.e. xhr), you should specify the content type header on the request as "application/json; charset=yourcharset".

I believe the above solution always returns json, no matter what the content type is specified from the client. The dotnet asmx framework allows this using the content-type header method, so the above could be classified as a hack, when a neat solution is available.

Similar question at Return Json Data from ASMX web service

This might help too -> http://forums.asp.net/p/1054378/2338982.aspx#2338982

P.S: I am assuming you are using dotnet version 4.

Question 4

Sometimes we don't have access to make changes in WebService definition and sometimes we should not make changes in WebServices! so the best solution is set contentType="application/json; charset=..." in your request. just this!

For example you can use below code (if you are using jquery) instead of changing somethings in WebService:

$.ajax({
        type: "POST",
        url: '/ServiceName.asmx/WebMethodName',
        date: {},
        contentType: "application/json; charset=utf-8",
        success: function (data) {                
                // Some code;

        }
    });

Then you can use JSON.parse(data.d) to access json structure of your data.