Switching MVC view on Post back using strongly typed views/view models

Question

User requests page for Step1, fills out and submits form that contains selected person, so far so good. After validation of ModelState the next viewmodel is constructed properly using the selected person. I then attempt a redirect to action using the newVM but find on entry to Step2 that MVC wipes out the viewmodel attempted to be passed in. I suspect this is due to how MVC attempts to new up and instance based on query string results. I'll put a breakpoint in and check that, but am wondering how does one change a view from a post back with a new view model passed in?

public ActionResult Step1()
{
    var vm = new VMStep1();
    return View(vm);
}

[HttpPost]
public ActionResult Step1(VMStep1 vm)
{
    if (ModelState.IsValid)
    {
        var newVM = new VMStep2(vm.SelectedPerson);
        return RedirectToAction("Step2", newVM);
    }
    return View(vm);
}

public ActionResult Step2(VMStep2 vm)
{
    return View(vm);
}

I can fix this by containing VMStep2 and a partial to Step2 in Step1 view, but that requires hide and seek logic when really I just want user to see Step2.

Answer 1

I don't see why you should want to call RedirectToAction! What it does it the following: it tells your browser to redirect and like it or not your browser doesn't understand how to handle your object -- what it does understand is JSON. So if you really insist on using return RedirectToAction("Step2", newVM); you should consider a way to serialize your VMStep2 object to JSON and when the browser requests the Redirect, it will be properly passed and created in your action method public ActionResult Step2(VMStep2 vm)

HOWEVER I'd use a much simpler way --- instead of

return RedirectToAction("Step2", newVM);

I would use

return View("Step2", newVM);