MVC Controller post json does not work - ajax

Question

I have the this code to post json to a controller.
The problem is that the credentials object does not get populated with the posted values.
How do I change this code so that it works?
I see in Fiddler that the request is being posted correctly.
[HttpPost]
public JsonResult Authenticate(CredentialsModel credentials)
{
return Json(credentials);
}
[DataContract]
public class CredentialsModel
{
[DataMember(Name = "user")]
public string User;
[DataMember(Name = "pass")]
public string Pass;
}
$.ajax({
type: "POST",
url: "/login/authenticate",
cache: false,
contentType: "application/json; charset=utf-8",
data: '{"user":' + JSON.stringify($('#username').val()) + ',"uass":' + JSON.stringify($('#userpass').val()) + '}',
dataType: "json",
timeout: 100,
success: function (msg) {
},
complete: function (jqXHR, status) {
if (status == 'success' || status == 'notmodified') {
var obj = jQuery.parseJSON(jqXHR.responseText);
}
},
error: function (req, status, error) {
}
});

Answer 1

The default MVC model binder only works with properties. Your CredentialsModel is using fields. Try changing them to properties. You can also remove the annotations.
public class CredentialsModel
{
public string User { get; set; }
public string Pass { get; set; }
}
Also, as pointed out by Sahib, you can create a Javascript Object and then stringify it, rather than stringifying each one. Although that doesn't appear to be the problem in this case.
data: JSON.stringify({
User: $('#username').val(),
Pass: $('#userpass').val()
})

Answer 2

Try chaning your data like this :
$.ajax({
.................
//notice the 'U' and 'P'. I have changed those to exactly match with your model field.
data: JSON.stringify({User: $('#username').val(),Pass: $('#userpass').val()}),
.................
});