Posting JSON data via jQuery to ASP .NET MVC 4 controller action

The problem is your dataType and the format of your data parameter. I just tested this in a sandbox and the following works:

C#

    [HttpPost]
    public string ConvertLogInfoToXml(string jsonOfLog)
    {
        return Convert.ToString(jsonOfLog);
    }

javascript

<input type="button" onclick="test()"/>

    <script type="text/javascript">

        function test() {
            data = { prop: 1, myArray: [1, "two", 3] };
            //'data' is much more complicated in my real application
            var jsonOfLog = JSON.stringify(data);

            $.ajax({
                type: 'POST',
                dataType: 'text',
                url: "Home/ConvertLogInfoToXml",
                data: "jsonOfLog=" + jsonOfLog,
                success: function (returnPayload) {
                    console && console.log("request succeeded");
                },
                error: function (xhr, ajaxOptions, thrownError) {
                    console && console.log("request failed");
                },

                processData: false,
                async: false
            });
        }

    </script>

Pay special attention to data, when sending text, you need to send a variable that matches the name of your parameter. It’s not pretty, but it will get you your coveted unformatted string.

When running this, jsonOfLog looks like this in the server function:

    jsonOfLog   "{\"prop\":1,\"myArray\":[1,\"two\",3]}"    string

The HTTP POST header:

Key Value
Request POST /Home/ConvertLogInfoToXml HTTP/1.1
Accept  text/plain, */*; q=0.01
Content-Type    application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With    XMLHttpRequest
Referer http://localhost:50189/
Accept-Language en-US
Accept-Encoding gzip, deflate
User-Agent  Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0)
Host    localhost:50189
Content-Length  42
DNT 1
Connection  Keep-Alive
Cache-Control   no-cache
Cookie  EnableSSOUser=admin

The HTTP POST body:

jsonOfLog={"prop":1,"myArray":[1,"two",3]}

The response header:

Key Value
Cache-Control   private
Content-Type    text/html; charset=utf-8
Date    Fri, 28 Jun 2013 18:49:24 GMT
Response    HTTP/1.1 200 OK
Server  Microsoft-IIS/8.0
X-AspNet-Version    4.0.30319
X-AspNetMvc-Version 4.0
X-Powered-By    ASP.NET
X-SourceFiles   =?UTF-8?B?XFxwc2ZcaG9tZVxkb2N1bWVudHNcdmlzdWFsIHN0dWRpbyAyMDEyXFByb2plY3RzXE12YzRQbGF5Z3JvdW5kXE12YzRQbGF5Z3JvdW5kXEhvbWVcQ29udmVydExvZ0luZm9Ub1htbA==?=

The response body:

{"prop":1,"myArray":[1,"two",3]}