JSONP - Access-Control-Allow-Origin and MIME type errors

I am trying to both create the Server and client side of a jsonp request, and I cannot seem to make it work. I've look开发者_运维技巧ed around at numerous guides, blog posts, etc, but pretty much everyone shows only the client side.

Here's my client code

$.ajax({
    dataType: 'application/json',
    data: params,
    jsonp: 'jsonpCallback',
    url: settings.domain + '/httpext.dll?json_cc',
    success: function (data) {
        //determine the return status
    },
    error: function (response, status, error) {
        //error handling
    }
}); //end ajax

right now, the server is returning a hard coded value like this

jsonpCallback({"username":"meltingice","posts"1234});

My problem is that I cannot get the request and response to work together. Currently, the response is returning application/json, so if I change my request ro expect jsonp it errors with

Resource interpreted as script but transferred with MIME type application/json. Uncaught ReferenceError: jsonpCallback is not defined

First off, as you can see, I have jsonpCallback defined.

Now, if I change the dataType to application/json, then I get this error

XMLHttpRequest cannot load http://myserver/httpext.dll?json_cc&sid=adsfhasjdkfhajksdghjk%3Basdhg&action=SALE&ccCard=&ccNum=&ccExMM=0&ccExYYYY=0&ccCVV2=&holdersName=&totalDue=0&dueDate=11%2F19%2F2010. Origin http://localhost:59905 is not allowed by Access-Control-Allow-Origin.

As you can see, it is not putting callback=? into the url. It's rather frustrating.

How do I set up the server side so I can call it using jsonp? What do I need for the response type? How do I format the data returned so my client side code can pull back the data?

The reason this doesn't work is because your server is always returning a hardcoded method name and your client is using an anonymous success handler. This cannot work. You need to configure your server to use the method name passed as argument or it will never invoke the success callback at the client. So once you configure your server to take into account the jsoncallback query parameter you could test it like this:

$.getJSON(settings.domain + '/httpext.dll?json_cc&jsoncallback=?', params, function(result) {
    alert('success');
});

Now the server will receive a request that might look like this:

http://foo.com/httpext.dll?json_cc&jsoncallback=jsonp1290183992345

and it should respond like this:

jsonp1290183992345({"username":"meltingice","posts"1234});

with proper Content-Type as well obviously (application/json).

I also wanted to contribute this snippet I used in ASP.NET MVC to handle the jsoncallback.

    private static ActionResult GetContent(object data, string callback)
    {
        if (!string.IsNullOrEmpty(callback))
        {
            ContentResult result = new ContentResult();
            result.ContentType = "application/json";

            JavaScriptSerializer serializer = new JavaScriptSerializer();
            string json = serializer.Serialize(data);
            result.Content = string.Format("{0}({1})", callback, json);
            return result;
        }
        else
        {
            JsonResult result = new JsonResult();
            result.Data = data;
            result.JsonRequestBehavior = JsonRequestBehavior.AllowGet;
            return result;
        }
    }