Does MVC remember checkbox values after submit? - asp.net-mvc

Question

I'm a php head and getting to grips with ASP.NET MVC 5.
In php, after submitting checkbox fields, in order to post back the form with the checkboxes you checked initially set to true, you have to run a if isset on em.
However reading up on model binding in mvc, it seems that this is done automatically for you, i.e checkboxes are returned after form submit checked, if originally selected, using either the HTML.CheckBox or HTML.CheckBoxFor helpers.
Is this the case, or am I expecting too much of MVC?

Answer 1

No, ASP.NET MVC doesn't remember checkbox values after they're submitted. Being an HTTP application as soon as ASP.NET MVC has rendered the HTML it ends the request and forgets everything about what it's just done. Then upon submitting a form ASP.NET MVC processes the incoming HTTP request and maps it to your model via its model binding (more on how it does this in a moment).
Having come from a PHP background myself this is one of those questions I always had when starting with ASP.NET MVC.
With ASP.NET MVC you have to remember that you're working within the context of a complete framework, and in order to ensure you're as productive as possible ASP.NET MVC will take care of a lot of the mundane work for you - ASP.NET MVC's model binding is a perfect example of this.
When submitting a form, the ASP.NET MVC framework will parse all incoming post data and attempt to automatically map it to the values you're providing it via your controller action.
So where as in PHP you'd normally do something along the lines of:
if(isset($_POST['checkboxValue'])) {
$checkboxVal = $_POST['checkboxValue'];
}
ASP.NET MVC will automaltically bind the incoming post data to your action parameter like so:
[HttpPost]
public ActionResult Submit(bool checkboxValue){
}
It does this by checking the parameter name (checkboxValue) matches that of the post data array key, and that the type also matches up. For instance, if you were to change the above checkboxValue from a boolean to a string and change the name, then ASP.NET MVC's model binding will be unable to match the property to the post data and will not automatically set the value for you.
It's also worth noting that ASP.NET MVC's model binding doesn't know how you created the checkbox.
The HTML.CheckBox and HTML.CheckBoxFor html helpers are purely a means to make it easier for you to create the HTML. If you were to manually write the HTML yourself then the model binder will still successfully bind the submitted data.
Edit:
As #DrinkBird has quite rightly pointed out, you're also able to access all of your form's post data by using the FormCollection instance like so:
[HttpPost]
public ActionResult Submit(FormCollection postData){
}
This collection represents all of the data posted to the Submit action.

Answer 2

Yes, model-binding should allow you to retrieve the value of a checkbox on submission.
if your model looks like:
public class myModel
{
public bool myBool {get; set;}
}
and in your HTML, you've used the helper
#Html.CheckBoxFor(m => m.myBool)
Then in your post action to handle the submission:
[HttpPost]
public ActionResult MyAction(myModel model)
{
var whatsThis = model.myBool;
}
...whatsThis will be true if the checkbox was checked, false if not.
Part of why this works is that when you use #html.CheckBoxFor, it also places a hidden form field that will pass false if the box is unchecked, to aid with model binding -- if it didn't, per HTTP there would be no varibalbe 'myBool' submitted in the post-vars collection.
If you return this model back into the form (say, if it didn't validate), then the form will re-present the checkbox in whatever state it was in on submission:
[HttpPost]
public ActionResult MyAction(myModel model)
{
if(!ModelState.IsValid)
{
return View(model);
}
else
{
//do success
}
}