Question

In ASP.NET MVC the property ViewBag is of type dynamic.

Throught the code, various “properties” of ViewBag are assigned to:

ViewBag.Message = "Hello world";
ViewBag.Title = "Page title";

How does this work? From reading the docs I get that ViewBag itself can by anything, but why (or how) does assigning something to a “member” of dynamic type work?

Here is how ViewBag is defined in public abstract class ControllerBase : IController class:

 [Dynamic]
 public dynamic ViewBag { get; }
 //
 // Summary:
 //     Gets or sets the dictionary for view data.
 //
 // Returns:
 //     The dictionary for the view data.
 public ViewDataDictionary ViewData { get; set; }

Just curious.

Relevant question: Where can I find the official documentation for DynamicViewDataDictionary?

Answer 1

ViewBag is an ExpandoObject or something similar. This is basically just a glorified Dictionary with the “properties” being the keys.

UPDATE:
Actually, it is a DynamicViewDataDictionary, a class internal to the ASP.NET MVC source code. This class inherits from DynamicObject and simply is a wrapper around a ViewDataDictionary, in case of the ViewBag around the ViewData property of ControllerBase.
As you can see in TryGetMember and TrySetMember the calls are delegated to the ViewData without any logic.