Storing objects (non-string things) in TempDataDictionary does not round trip correctly

[pranavkm]

Scenario

public class FeedbackController : Controller
{
    private const string _userKey = "user-data";

    [HttpPost]
    public IActionResult SubmitApplication(string name, int age)
    {
        // Do db stuff
        var user = new User { Id = 100, Name = name, Age = age };
        TempData[_userKey] = user;
        return RedirectToAction("WelcomeUser");
    }

    public string WelcomeUser()
    {
        var user = (User)TempData[_userKey];
        return "Welcome " + user.Name;
    }
}

Expected (per behavior in Mvc 5)
Ouputs Hello {userName}

Actual
Unable to cast object of type 'Newtonsoft.Json.Linq.JObject' to type 'User'.

[pranavkm]

It looks like we Json serialize values in TempDataDictionary as opposed to using binary serialization in Mvc 5. Consequently complex types don't deserialize correctly. Perhaps we should consider disallowing storing non-primitive value types + strings in our default implementation if the (totally reasonable) intention is to avoid binary serialization.

[pranavkm]

Minor note: In-memory session state is simply holding on to the object in memory, not serializing them. Consequently, you can store objects that cannot be deserialized in it

public class User
{
   public User(int id) {}
   public string Name { get; set; }
}

This is to counter @danroth27's suggestion to use Json.Net's type name handling behavior to serialize types.

[danroth27]

@pranavkm Consider restricting to only primitives, dictionaries of primitives and arrays of primitives for now.

[pranavkm]

I think this was meant to be assigned to @ajaybhargavb. Let's talk about this before you start working on it.