How to handle a strongly typed modal in a strongly typed view with different types

Question

There is an overload in @Html.Partial that takes an object, used for the partial page's Model. If you include the Partial in your layout, in every page, you need a logic to keep that data. For example, if you take LoginModel and RegisterModel, you could do this:

@Html.Partial("_LoginPartial", ViewBag.LoginModel ?? new LoginModel())
@Html.Partial("_RegisterPartial", ViewBag.RegisterModel ?? new RegisterModel())

And leave to the executing controller the role to put a LoginModel (or RegisterModel). If there isn't any in the ViewBag, it will fallback to creating an empty one.

Edit: Based on the additional information, I'd do this for the LoginPartial (RegisterPartial would be the same logic):

public class AccountController : Controller
{
    public ActionResult LoginPartial()
    {
        return PartialView("_LoginPartial", (Session["Login"] as LoginModel) ?? new LoginModel());
    }

    [HttpPost]
    public HttpStatusCodeResult SaveLoginModel(LoginModel model)
    {
        Session["Login"] = model;
        return new HttpStatusCodeResult(200);
    }
}

And then, in _LoginPartial, do as you would, but add a javascript code to send an ajax post request to the SaveLoginModel action of your controller when the value change to keep your model in sync (there is plenty of information around on how to do that).

Now, instead of doing:

@Html.Partial("_LoginPartial", ViewBag.LoginModel ?? new LoginModel())
@Html.Partial("_RegisterPartial", ViewBag.RegisterModel ?? new RegisterModel())

You would do:

@Html.Action("LoginPartial", "AccountController");
@Html.Action("RegisterPartial", "AccountController");