How does ViewBag in ASP.NET MVC work behind the scenes?

Question

I am reading a book on ASP.NET MVC and I'm wondering how the following example works:

Example #1

Controller

public class MyController : Controller
{
    public ActionResult Index()
    {
        ViewBag.MyProperty = 5;

        return View();
    }
}

View

<h1>@ViewBag.MyProperty</h1>

Now I understand that ViewBag is a dynamic object, so that's how you can set the property (though I don't know much about dynamic objects, never worked with them.) But how does the view get the specific instance of the ViewBag from the controller, even though we don't pass anything directly?

I thought that the ViewBag could be a public static object, but then any change to it would be global and it wouldn't be specific to a view instance.

Could you elaborate as to how this works behind the scenes?

Example #2

Controller

public class MyController : Controller
{
    public ActionResult Index()
    {
        ViewBag.MyProperty = 5;

        return View();
    }

    public ActionResult Index2()
    {
        ViewBag.MyProperty = 6;

        return View();
    }
}

Now let's say the Index method is called first, and then the Index2. In the end the value of ViewBag.MyProperty will end up as 6 (the value from Index2). I feel that it is not a good thing to do, but at the same time I feel that I'm thinking in desktop development terms. Maybe it doesn't matter when used with ASP.NET MVC, as the web is stateless. Is this the case?

Answer 1

ViewBag is a property of ControllerBase, which all controllers must inherit from. It's a dynamic object, that's why you can add new properties to it without getting compile time errors.

It's not static, it's a member of the object. During the request lifetime, the controller instance is created and disposed, so you won't have "concurrency" problems, like overwriting the value.

The View (and its variants) method is not static as well, and this is how the view receives the ViewBag values: during the process of rendering the view, the controller instance has its ViewBag instance as well.

Answer 2

If you would analyse ControllerBase class you would see that ViewBag property is a "proxy" to ViewData property just to make your source look nicer. (I even remember Scott Hanselman taking interview from Phil Haack where Phil introduced ViewBag property as a shortcut to ViewData and eliminating the need of repeated square brackets and quotes). Even though ViewBag property is exposed as dynamic object it implements a DynamicViewDataDictionary class which works directly with ViewData.

Looking at source code of Controller class you can find this method:

protected internal virtual ViewResult View(string viewName, string masterName, object model)

So basically when you call return View(); from your controller it creates a new instance of ActionResult class passing ViewData from controller to it's constructor. Instance of ActionResult is then passed to a particular view engine (ASPX, Razor) so it could be used to render a view in question.

Making ViewBag/ViewData public static could be harmful. Each web request to your MVC application creates a new instance of controller. If you'd have ViewData/ViewBag as public static then two concurrent users would share same data from ViewBag/ViewData.

Here is a video. Discussion on ViewBag (formder ViewModel) starts at 04:05

Answer 3

ViewBag is a property of ControllerBase. It is defined as follows:

public Object ViewBag { get; }

Note that this signature is actually incorrect. Here's what the source code actually looks like:

public dynamic ViewBag {
        get {
            if (_dynamicViewDataDictionary == null) {
                _dynamicViewDataDictionary = new DynamicViewDataDictionary(() => ViewData);
            }
            return _dynamicViewDataDictionary;
        }
    }

_dynamicViewDataDictionary is an ExpandoObject; you can add properties to it at runtime. Its lifetime is the same as that of the controller, which is the lifetime of the HTTP request.