Jquery post to mvc 4 controller not passing data - asp.net-mvc

Question

I'm trying to pass data from JQuery to an MVC 4 controller. The controller gets invoked, but no data is passed. In the past I always just used form serialization, but that's not appropriate here.
My Controller:
[HttpPost]
public ActionResult Write(VideoSessionEnvelope envelope)
{
if (ModelState.IsValid)
{
envelope = Log.Write(envelope);
}
var result = Json(envelope);
return result;
}
We use an envelope class as a container for all view models
public class VideoSessionEnvelope : BaseEnvelope
{
public VideoSessionEnvelope()
{
SessionStart = new VideoSessionStartViewModel();
}
public Guid? LogEntryID { get; set; }
public VideoSessionStartViewModel SessionStart { get; set; }
}
}
The view model
public class VideoSessionStartViewModel: IViewModel
{
public string SessionId { get; set; }
public int UserId { get; set; }
public string Message { get; set; }
}
And finally the javascript
var Logging = Logging || {};
Logging.VideoSession = function () {
var Start = function (sessionId, userId, message) {
var envelope = {
SessionStart: {
"SessionId": sessionId,
"UserId": userId,
"Message": message
}
}
var data = JSON.stringify(envelope);
$.ajax({
type: "POST",
url: "/Logging/Write",
data: data,
datatype: "application/json",
success: function (result) {
return result;
},
error: function (request, status, error) {
return error;
}
});
};
return {
Start: Start
};
}();
According to Firebug the data is passed as
JSON
SessionStart Object { SessionId="sessionIdVal", UserId=123, Message="messageValue"}
Message "messageValue"
SessionId "sessionIdVal"
UserId 123
The controller gets called, but the properties in the view model are always null. I've tried several variations on the theme, nothing seems to work.

Answer 1

Try wrapping your data in a literal with the name as envelope so it will be picked up by the Model Binder:
data: { envelope: data },
UPDATE
Remove the call to JSON.stringify(), it is not strictly necessary to serialize the object literal.