Normally I don’t do this, but since this is such a great article:
http://blog.getpaint.net/2008/12/06/a-fluent-approach-to-c-parameter-validation/
From the creator of Paint.NET.
It’s really worth reading, if you do a lot of parameter validation. It might come in handy for your forms, or maybe even for a website.
I think I’ll write something based on that really soon 