The ASP.NET 2.0 CSS Friendly Control Adapters 1.0 are fantastic tools.  Really amazing; you simply drop in 30 pre-built files into your application and your TreeView, Menu, GridView and various login controls stop spitting out table-based layouts and start generating CSS-compliant code.  Oh yeah, it's fantastic.

So here I am, happily pounding out CSS formatting, dropping my login controls building, and merrily moving along my way when I dropped in the TreeView control, went to view the new page and WHAMMY!

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

Love affair over. 

Far too many hours later I was able to determine that a very small piece of code on my master page was the root of the TreeView's problems.  I had a footer that was simply writing some standard copy along the lines of, "this site ©2007 My Company" and was dynamically generating the 2007 with the following code:

<%= DateTime.Now.Year.ToString() %>

Removing that code resulted in a running page.  Although removing this code and replacing it with a Label object would be a solution, I have a bad feeling this bug would come back to haunt me some time in the future.

Luckily, I found a solution buried in the forums at ASP.NET, specifically this one from Dmitry Karpenkov dated 4/4/2007.  Add a private string, a method for it, and change the SaveAdapterViewState() method for the TreeView and you're back to work.  I do need to really inspect the fix to find out if there are any negative results, but for now, I'm moving on.