An example of how to use Precondition Filters in conjunction with HandleErrorAttribute to provide user-friendly error pages when users try to link to your URLs with invalid route data, query strings, etc...

An updated version of ASP.NET MVC Precondition Filter that is now part of MVCContrib ...

Here is code that will allow you to decorate your MVC Actions with attributes that will define your needed "preconditions". I often find the need to check for the existence of a parameter passed to a web page, such as an ID, and also often need to check if the ID is valid as well (e.g.: a valid primary key). These, I consider "preconditions".