Getting a serialized object from a webservice

Question

I am coding a C# Web API 2 webservice where I am getting a simple object with some AJAX code. The object is being returned correctly, however, if the exact same object has a [Serializable] attribute, then the AJAX code is not working correctly.

Here is the simple object:

public class Simple
{
    public int number { get; set; }
    public string data { get; set; }
}

Here is the AJAX code:

function sendRequest() {
    $.ajax({
        type: 'GET',
        url: serviceUrl2,
            beforeSend: function (xhr){ 
            xhr.setRequestHeader('Authorization', make_base_auth(userName, password)); 
        }
    }).success(function (data) {
        alert('Success');
        alert('data:' + data.data);
    }).error(function (jqXHR, textStatus, errorThrown) {
        $('#value1').text(jqXHR.responseText || textStatus);
    });
}

The above code works correctly, and an alert is shown with the data.

If the same object is coded as follows:

[Serializable]
public class Simple
{
    public int number { get; set; }
    public string data { get; set; }
}

The above code does not work correctly, and an alert is shown with "data:undefined"

Here is the Web API function:

[System.Web.Http.HttpGet]
[Route("Getsimple")]
[ResponseType(typeof(Simple))]
public async Task<IHttpActionResult> GetSimple()
{
    if (!User.Identity.IsAuthenticated)
    {

    }
    var simple = new Simple();
    simple.number = 1;
    simple.data = "test data";
    return Ok(simple);
}

Do I need to add a specific parameter to the AJAX code or do I need to serialize the Simple object before the object is returned? If not, how can I get the code working?