How to show custom validation error message from Tridion Event Handler code in popup window instead of Message Center

Question

I would like to show a custom validation error message from the Event Handler code in a popup window.

My requirement is while saving binary Component; if the binary filename contains any special character, then show the error message in a popup window instead of the Message Center.

For filename validation, I thought of adding the validation in Tridion Event Handler code. But not sure how we can display the error in a popup window?

Answer 1

I wrote a working example of that here: "Validating Content on Save, Part 1".

Part 2 and 3 were published by Robert Curlette here: http://www.curlette.com/?p=913