Validation controls perform input checking in server code. When the user submits a page to the server, the validation controls are invoked to check the user input, control by control. If a validation error is detected in any of the input controls, the page itself is set to an invalid state so you can test for validity before your code runs. Validation occurs after page initialization (that is, after view state and postback data have been processed) but before any change or click event handlers are called.

If the user is working with a browser that supports ECMAScript (JavaScript), the validation controls can also perform validation using client script. This can improve response time in the page because errors are detected immediately and error messages are displayed as soon as the user leaves the control containing the error. If client-side validation is available, you have greater control over the layout of error messages and can display an error summary in a message box