Update: If you are curious as to why MS added the .d attribute, find out why at Encosia.

I was very frustrated the other day trying to figure out why a jQuery ajax call worked on my dev box but not on the server.  It looked something like this:

json(_serviceUrl, "{}", true,
    function(result) { fillSelect($("#ddlDepartment")[0], result.d); },
    function(ajax) { /* handle error */ });
    
// calls a json web service
function json(url, data, async, onSuccess, onFailed)
{
    $.ajax({
        async: async,
        type: "POST",
        url: url,
        data: data,
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: onSuccess,
        error: onFailed
    });
}

(The code inside the json function is straight from Dave Ward's post on Using jQuery to Consume ASP.NET JSON Web Services. Dave's blog is an excellent resource full of information on jQuery, AJAX, ASP.Net and how to make them play nice together.)

I figured out the problem was result.d; on the server it was null, but on my machine it was a ListItem[], as expected. It turns out that on my machine, the web service at _serviceUrl was compiled against .Net 3.5. while on the server it was compiled against .Net 2.0 with ASP.NET AJAX Extensions 1.0. In order to get it working on the server, I had to change result.d to result. Apparently they changed a few things in System.Web.Extensions v3.5. Unfortunately, I'm unable to install .Net 3.5 on the server. So I came up with this helper function to get things working in both 1.0 and 3.5:

// gets the ajaxResult. Returns ajaxResult.d is if it is not null, else ajaxResult.
// System.Web.Extensions v3.5 web services will result in ajaxResult.d while v1.0 will be ajaxResult.
function getResult(ajaxResult)
{
    return ajaxResult.d == null ? ajaxResult : ajaxResult.d;
}

So now after replacing result with getResult(result), the json call looks like this:

json(_serviceUrl, "{}", true,
    function(result) { fillSelect($("#ddlDepartment")[0], getResult(result)); },
    function(ajax) { /* handle error */ });

Hopefully this will save someone out there some frustration!