ePortal automatically includes validation as part of the message field runtime error checking. Refer to Validation Performed by HTML5-Compatible Browsers and Validation Performed by ePortal for more information. You can also add validation to perform checks that ePortal does not perform and to have more control over how errors are displayed on the page.
ASP.Net contains validation controls expressly designed for this purpose. To use a validation control, do the following:
Open the .aspx file in the editor (refer to Opening the Generated Web Forms Files).
Select the Design tab.
Open the Toolbox window and expand the category Validation.
Drag and drop one or more validation controls onto your Web page.
By default, the validation control will display a validation error message, if applicable, at the point where you place it on your Web page.
By default, ASP.Net validation controls validation in the browser, using JavaScript, where possible. In addition, validation is performed on the web server system for the cases in which browser validation is not possible.
For detailed information on using ASP.Net validation controls, search for the topic "Validation ASP.Net Controls" in the MSDN online documentation.
Refer to Using a CustomValidator with Message Field Runtime Error Checking.