Simply put, never trust the data in an incoming request, and always validate on the server.When validation fails the normal flow of execution continues.As we mentioned earlier, validation controls execute when the user clicks a Button control, including HTML button controls and server button controls such as the Link Button and Image Button.

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We have seen the Required Field Validator in our first sample program, let’s move on to examine the rest of the controls.

The Range Validator control ensures the value inside a control is within a specified range.

The cancel button has a Causes Validation property. As we mentioned in the beginning of the article, validation can occur twice: once on the client’s machine and once on the server. Client side validation provides quick feedback for the user.

Validation occurs when a user clicks any Button control by default, but you can change this behavior by setting the Causes Validation property to false. Whenever the user presses a button on the form, the script executes the validation checks for each validation control on the form.

One of the features of HTML5 is the ability to validate most user data without relying on scripts.

This is done using validation attributes on form elements.

Before accepting data from over the network you need to ensure all of the required fields have been filled out and conform to your formats and business rules. NET provides a number of validation controls to ease the burden of performing validation on both sides of the network. To walk along with the example, create a new web application in Visual Studio . Once the application is setup, right click on the empty web form, select Properties from the context menu, and change the Page Layout property to Flow Layout. Notice we need to check the Is Valid property of the base class before performing any critical action in the submit event.

Place four controls from the Web forms section of the Toolbox onto the form. If Is Valid returns a value of true then all of the validation checks were successful.

While you should always validate data on your servers, additional validation of data on the Web page itself has multiple benefits. By validating form data while the user is filling it out, the user can know immediately if they've made any mistakes; this saves the time of waiting for an HTTP response and saves your server from dealing with bad form input.

This article covers how to validate form data within your form's Web content.

Drag one Text Box control and give it an ID of First Name Text Box, two Button controls (Submit Button and Cancel Button), and one Required Field Validator (First Name Required Field Validator). The following screen shows the form in action, and the user has pressed the Submit button without entering any text into the textbox. The Required Field Validator fails when the value in the associated control matches the value of the Initial Value property of the Required Field Validator.