Question

I am just getting started with ASP.Net MVC 3 and am confused on this point.

In some examples when an action in a controller is run that contains inputs then a check is made to ensure ModelState.IsValid is true. Some examples do not show this check being made. When should I make this check? Must it be used whenever an input is provided to the action method?

Answer 1

Must it be used whenever an input is provided to the action method?

It is exactly when you are using a view model provided as action argument and this view model has some validation associated with it (for example data annotations). Here’s the usual pattern:

public class MyViewModel
{
    [Required]
    public string Name { get; set; }
}

and then:

[HttpPost]
public ActionResult Foo(MyViewModel model)
{
    if (!ModelState.IsValid)
    {
        // the model is not valid => we redisplay the view and show the
        // corresponding error messages so that the user can fix them:
        return View(model);
    }

    // At this stage we know that the model passed validation 
    // => we may process it and redirect
    // TODO: map the view model back to a domain model and pass this domain model
    // to the service layer for processing

    return RedirectToAction("Success");
}