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".