Question

Here is one of the examples that I’ve seen on how to do validation on Controller:

[HttpPost]
public ViewResult Create(MyModel response)
{
    if (ModelState.IsValid)
    {
        return View("Thanks");
    }
    else
    {
        return View();
    }
}

If there are validation errors, than return View() method is called without any parameters. Obviously you have @Html.ValidationSummary() in your View and Model has all required property attributes.

The data that was entered into the form was preserved and displayed again when the view was rendered with the validation summary.

My question: how is the data preserved? Since it was not passed to the View like

return View(response);

Thanks a lot.

Sincerely,
Vlad

Answer 1

It is because the values have been bound to the model state which is passed back along to the view. This is along the same issue/question on why you cannot change a model value and return the view again. Meaning, let’s assume I have the following property on my Viewmodel

public string Name {get;set;}

Using the controller below, I cannot change the ViewModel property without also either clearing the ModelState or updating the value in the model state. Try it!

[HttpPost]
public ViewResult Create(MyModel response)
{
    response.Name = response.Name + "Some Random String"
    return View();
}

The name property will remain unchanged. Essentially, once the ModelBinding occurs, the values from your form (ViewModel) are bound to the model state, which is why you do not have to pass the model back to the view.

As a side note, I always pass the model back in my call to return View();, it just seems more correct and a little easier to read