Question

Basically, I want to inject some data into ViewData/ViewBag for every single request.

Answer 1

In ASP.NET MVC that would be an action filter. And if you want to do it globally you could register it as a global action filter. This way it will apply to all controller actions so that you don’t need to decorate them individually.

So your filter could be defined like this:

public class GlobalViewBagInjectorActionFilter : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        filterContext.Controller.ViewBag.Foo = "bar";
    }
}

and registered in the RegisterGlobalFilters method in your Global.asax:

public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
    filters.Add(new HandleErrorAttribute());
    filters.Add(new GlobalViewBagInjectorActionFilter());
}

Now inside all your views you can use the ViewBag.Foo property.

But in most situations Child Actions are a better alternative than ViewBag as they allow you to pass strongly typed view models instead of relying on this weakly typed ViewBag and some magic strings.