I just upgraded my site from ASP.NET 3.5 to 4.0. I've been working through some breaking changes and there were more than I expected.
One I can't figure out, however, is why my <asp:Substitution /> control suddenly stopped working like it should. It's supposed to ignore the output cache settings of the parent page and update upon every request. For some reason that isn't happening. It's caching for the full 10 minutes (the OutputCache setting for my home page). Any ideas?
<asp:Substitution> and other server controls are not supported in MVC. Response substitution in particular is intricately tied to the WebForms pipeline. The fact that it worked in MVC 1 was a happy accident but was not intentional.
The MVC team is working on ways to enable substitution caching in MVC 3 and beyond, but there will likely be MVC-specific mechanisms for doing this, so you shouldn't expect <asp:Substitution> or Response.WriteSubstitution() to work going forward. In the meantime, the best option is to output cache the entire page, then use Javascript / AJAX to have the client fill in the parts that need to be updated.