ASP.NET validation controls (also known as validators ) greatly simplify the task of ensuring that data is entered correctly on forms. For most validations, no code is required in either the .aspx file or the code-behind class. You simply add a validator to the .aspx file, have it reference an input control (a server control) elsewhere on the page, and set one or more of its validation attributes (such as MinimumValue or MaximumValue , which specify the minimum and maximum values of a validation range). ASP.NET does all the rest. You can also combine validators to provide multiple validations on a single input, such as a RequiredFieldValidator and a RangeValidator , which perform as their names imply.
Validation can be performed on the client and the server. By default, validators perform their validation automatically on postback in server code. However, if the user has a browser that supports DHTML and client-side validation is enabled, validators can also perform their validation using client script. Client-side validation is handy whenever you want to avoid a round trip to the server for server-side validation, such as when you want to make sure an entry is provided in a text box. Regardless of whether client-side validation is performed, server-side validation is always a good idea, if only to ensure that validation always takes place, even when the user 's browser doesn't support DHTML.
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All validators, except the RequiredFieldValidator , allow the control being validated to be left blank. This is a subtle point but one worth noting, as you may need to account for this behavior in your code when using ASP.NET's automatic validation.