ASP.NET MVC Tip #1 - Accessing TempData from a ViewUserControl
Scenario:
I've been creating several shared components that I use throughout the MVC application. I found that almost all my forms needed some kind of feedback mechanism on whether the form was successful or not. I created an ErrorDisplay ViewUserControl which is called on most form input pages using the MVC HTML helper: <%= RenderUserControl("~/Views/Shared/ErrorDisplay.ascx") %>.
This simple control will check for a key in the TempData dictionary and if it finds it, it will loop through the items in that dictionary. The item I store in the TempData is a dictionary of error messages added from the controller. My issue in this case was that I kept getting a null reference exception when trying to access the TempData from the user control.
Solution:
On view pages, to access the TempData dictionary you can go through the ViewContext property directly. However, from a view user control, we have to access it through the Html.ViewContext.TempData property. This seems like a bug to me which I'm sure will be fixed in the next drop of the MVC framework. Also, I believe that TempData will also be accessible directly as a property of the View Page in future releases.
Hope this helps!