Whenever a button or other control would cause a page submission validation should occur. It's a little more complicated than that because validation can occur on both the client as well as on the server. If the user is working with a supported browser then many types of validation can be done on the client itself, which will significantly improve response time. For example, checking for required fields or that a value is in range can be done in JavaScript on the client without making a round trip to the server. ASP.NET automatically generates code to perform client side validation when a supported browser is being used. You can disable client side validation by setting the validation control's EnableClientScript property to False.

By using the client side validation we can intend only as a way to improve application response times by eliminating server round trips. Validation always occur on the server if a validation control is enabled. If client side validation precluded server side validation, then it would be possible for a malicious program to spoof client side validation and send bad data to the server.

On server, validation occurs after page initialization, but before any Change or Click events are processed. A page property, IsValid is set to indicate if validation was successful done. If IsValid is True, then you know that all of the input validation conditions were met, otherwise if IsValid is False, then you know that at least one validation failed. You have to check the IsValid property for each of the validators to determine which of the validators failed. On other hand you can use the page-level Validators collection to loop through all of the validators on a page.

To check input forms for errors and, if necessary, display messages to the user Microsoft ASP.NET provides a powerful validation mechanism that you can use with. ASP.NET mobile Web Forms provide mobile controls that correspond to each of the validation controls available in ordinary Web Forms pages. Each mobile control wraps the corresponding Web Form validator and provide an identical object model to its peer about the form. Thus, if you are familiar with the way validation server control work in Web Forms, you are already familiar with how to work with validation in mobile Web Forms pages. Mobile web page validation control work with a limited subset of mobile controls. For each control, a specific property contains the value to be validated. The following table shows the input controls that can be validated.

Use the RequiredFieldValidator to ensure that a response was supplied for an input field. The other validation controls work similarly. For example, suppose you want the user to input a number between 1 and 100. To validate that input, place a RangeValidator on the form, set its ControlToValidate property to the control that you want to validate, and set the MinimumValue and MaximumValue properties to 1 and 100, respectively. Set the ErrorMessage property to something descriptive such as "Please enter a number between 1 and 100." as the message(help). You would use a similar procedure for the CompareValidator and RegularExpressionValidator controls also.

You can use the ValidationSummary mobile control to display a summary of validation errors, instead of showing each validation error separately. Create a separate form on your page, place the control on it, and set its FormToValidate property to the form that you want to validate to use a ValidationSummary mobile control. Similar to its Web Forms equivalent, the ValidationSummary mobile control polls each of the validation controls on the given form and displays a summary of the results.