Question

I have an ajax POST request that is supposed to be submitting data to an ASP.NET MVC action. This works fine 95% of the time but every so often, for whatever reason, my FormCollection is empty and the application errors out as a result.

I would have included the raw request values however I haven’t been able to reproduce this myself.

My ajax call looks like this:

var data = {};
data.Property = 123; // etc

$.ajax({
    url: url,
    type: 'POST',
    dataType: 'json',
    data: JSON.stringify(data),
    success: function(viewModel) {
        // do stuff
    }
});

What would cause the FormCollection to intermittently be empty on the ajax POST?

I have suspected things like session timeouts and the like but I would expect that to error out before I get in to the action.

Answer 1

First of all you seem to be sending a JSON request (JSON.stringify(data)). This is not supported out of the box in ASP.NET MVC 2. It is in ASP.NET MVC 3 that a JsonValueProviderFactory was introduced. You may take a look at the following article which explains in more details JSON requests in ASP.NET MVC.

Also everytime you want to send a JSON request you should set the proper content type using the contentType parameter:

data: JSON.stringify(data),
contentType: 'application/json; charset=utf-8',

and last but not least I would recommend you to use view models instead of relying on some weakly typed structures such as FormCollection in your controller actions and having to write plumbing code.

---

UPDATE:

FormCollection is a key/value pair collection. So if you want to use it (despite my recommendation against it) you should not use a JSON request. You should use a normal key value request. So remove the JSON.stringify from your AJAX request and remove the contentType: ‘application/json’ parameter as well. There’s really no point in using weakly typed key/value pair collection with JSON objects.