It's a dynamic object, meaning you can add properties to it in the controller, and read them later in the view, because you are essentially creating the object as you do, a feature of the dynamic type. See this MSDN article on dynamics. See this article on it's usage in relation to MVC.
If you wanted to use this for web forms, add a dynamic property to a base page class like so:
public class BasePage : Page
{
public dynamic ViewBagProperty
{
get;
set;
}
}
Have all of your pages inherit from this. You should be able to, in your ASP.NET markup, do:
<%= ViewBagProperty.X %>
That should work. If not, there are ways to work around it.