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If you point the ControlToCompare attribute of the CompareValidator at a HiddenField you’ll get an error that looks something like this:
Control ‘hiddenBalance’ referenced by the ControlToCompare property of ‘CompareValidator1′ cannot be validated.

With a small amount of “googling” I found two useful articles:

Validating against the value in a hidden input
MS KB Article 310082

Between these two article [...]

The Google Maps API requires you to generate a key that is tied to the URL where the map will be deployed. This can be inconvenient if the site exists in multiple places at the same time…as is the case in many development scenarios:

Develop
Test
Deploy

These three stages can also represent physical locations. Development is [...]

Pre-requisites:

Atlas Framework
Atlas Control Toolkit

Summary
The Atlas Control Toolkit site does a good job a walking through all the steps necessary to build a custom extender. You’ll want to refer to that site for more general detail. I’m going to show you my own custom implementation.
I needed a control that would sum all the TextBoxes [...]

Responding to a recent comment by farmerbob, I decided to investigate The Mono Project. According to the website…

Mono provides the necessary software to develop and run .NET client and server applications on Linux, Solaris, Mac OS X, Windows, and Unix.

Excited about the prospect of building .NET applications on my Apple iMac, I [...]