The .NET Framework supplies a complete set of validation controls that you can use in your ASP.NET applications. From the simple RequiredFieldValidator control to the CustomValidator control, which allows you to supply both client- and server-side validation procedures, you have complete control over exactly what data you'll accept from your users. You no longer have to write complex scripting code, as you did when creating ASP applications! You'll want to dig into regular expressions a little, as well, because the ability to create your own expressions will cut down on the code you must write.
You can use validation controls to do the following:
Require input or ensure entered data is different from a default value by using the RequiredFieldValidator ...