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?

Erythrite

I would imagine that the ViewBag object is simply instantiated with the controller object as an instance property (not static). – Chronometry

You can see it's actually a member of the controller: msdn.microsoft.com/en-us/library/… – Baculiform

So if there are two methods returning different views set the same property on the ViewBag instance, it can affect each other? Wouldn't that cause a problem? I feel that I'm still thinking in desktop development though. – Erythrite

The controller is instantiated when Index() is called, and disposed after return View(); is executed. So ViewBag goes out of scope once the entire request is completed. That's why the ViewBag properties do not persist across requests. – Chronometry

@RobertHarvey, alright then. It makes sense now. I didn't know what was instantiated when. As I said, still thinking in desktop development way. Could you write these as an answer, I will choose it as the accepted answer. – Erythrite

I would, but it's unclear from the MSDN page that @dbaseman linked what is actually stored in that object property. I suspect that it is an ExpandoObject. – Chronometry

@Erythrite WebForms is a web framework that's close to desktop development. MVC, however, is way much different. It's much more the web paradigm. So, trying to understand MVC tools (such as the ViewBag) making a parallel to desktop will always lead you to confusing conclusions (or questions). – Tanney

I remember reading it was an ExpandObject couple minutes ago somewhere, let me see. – Erythrite

Wow, ExpandoObject is cool. It implements IEnumerable, so all of the Linq methods work on it. – Chronometry

@RobertHarvey, in this answer https://mcmap.net/q/203847/-how-viewbag-in-asp-net-mvc-works; Aniket says that it is an ExpandObject, but unfortunately he didn't cite his resource. @AndreCalil, you are right that it is completely different than desktop development. I'm just trying to wrap my mind around it and change my mindset. – Erythrite

You can browse the classes definition from VS: Controller inherits ControllerBase that has the public dynamic ViewBag { get; } property – Tanney

@RobertHarvey I believe that this question deserves an wiki answer =) – Tanney

There are so many things that works like magic behind the scenes of ASP.NET MVC though, it's not just trying to understand the way web works. So many conventions and magic stuff. It looks great but nothing that I'm used to. – Erythrite

@Erythrite I had this feeling when I started on ASP.NET MVC too, and I hate it. If I ask myself "how does this work?" and I can't give a reasonable answer, I'll look for it. Keep using MVC and you'll notice that there's no magic, it's all conventions (which you have to discover, somtimes). – Tanney

@AndreCalil, yeah sure, but it looks like magic until you read it and understand how it is implemented :) It's kind of cool though, I like it. It's just a bit tiring as I keep asking "Wait a minute, how the hell would that work?" all the time. – Erythrite

@Erythrite While learning ASP.NET MVC, nerddinner.codeplex.com helped me a lot. Check it out! – Tanney

@AndreCalil, thanks for the link. I have heard about it before and will definitely check it out. – Erythrite

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.

Tanney

ViewBag is a property of WebViewPage as well. how does it get data which is assigned to ControllerBase.ViewBag property. Can you elaborate please ? – Mercola

@Mercola the view receives it from the controller when the ActionResult is being created. If we take ASP.NET Core MVC as an example, you can see that ViewContext receives it on the ctor: github.com/aspnet/Mvc/blob/dev/src/… – Tanney

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

Accurate

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.

Chronometry

It appears the ViewBag property of ControllerBase has been consistently defined as dynamic, perhaps recently: public dynamic ViewBag { get; } – Gama

ViewBag is a property of WebViewPage as well. how does it get data which is assigned to ControllerBase.ViewBag property. Can you elaborate please ? – Mercola