Should you have separate view Models for input to and output from a controller

Question

I am new to asp.net mvc. I have this controller that takes in a few parameters and then returns a view that gets data based on the input parameters.

I want to accept the input parameters as an object (for example instead of first name, last name and age, I want a person class that has these three parameters as its properties). Now my question is does the input parameter class (Person class) qualify to be called view model? If yes. Do I make the return view model a part of this class?

In other words which of the bottom two approaches is preferred

Case 1: Same class for input and return

public ActionResult GetPersonDetails(Person p)
{

    return View(new Person {....})

}

Case 2: Separate classes for input and return

public ActionResult GetPersonDetails(Person p)
{

    return View(new PersonDetails {....})

}

Answer 1

Now my question is does the input parameter class (Person class) qualify to be called view model?

Yes.

If yes. Do I make the return view model a part of this class?

Not necessarily. You could have different view model passed to the view as the one that your controller action is taking as parameter, although this is rare case scenario. It would really depend on your specific case but the general pattern is the following:

[HttpGet]
public ActionResult Index()
{
    MyViewModel model = ...
    return View(model);
}

[HttpPost]
public ActionResult Index(MyViewModel model)
{
    if (!ModelState.IsValid)
    {
        // Some validation error occurred => redisplay the same view so
        // that the user can fix his errors
        return View(model);
    }

    // at this stage the view model has passed all validations =>
    // here you could attempt to pass those values to your backend

    // TODO: do something with the posted values like updating a database or something

    // Finally redirect to a successful action Redirect-After-Post pattern
    // http://en.wikipedia.org/wiki/Post/Redirect/Get
    return RedirectToAction("Success");
}