Asp.net textbox validate event

Question

I’m new in asp.net. and I’m am creating a simple Shoe Store Billing Management System. I want to know how to fire textbox validate event. Just like as we do in windows app textbox validate event. I have lost of shoes in my shoes table. And there is two column in table ShoseCode and ShoseDesc. When I enter the ShoseCode in ShoseCode Textbox and if this ShoseCode is already exist in ShoseCode Colum. So this ShoseCode information retrieve from database. Or if this cant retrieve so just show the message like “This Shoes Code already exist” or something like this.

Thanks in Advance J