Validation in ASP.NET refers to the process of checking user input data to ensure that it is valid and meets certain criteria. ASP.NET provides a list of validator controls that can be used to validate user input data, such as data format, data type, and date range. Some of the commonly used validation controls in ASP.NET include RequiredFieldValidator, CompareValidator, RangeValidator, RegularExpressionValidator, ValidationSummary, and CustomValidator.
Validation can be done on both the client-side and server-side. Client-side validation is done using JavaScript and is faster because it doesnt require a round-trip to the server. Server-side validation is done in the C# code-behind and provides better security.
In ASP.NET Core MVC and Razor Pages, model validation occurs after model binding and reports errors where data doesnt conform to business rules. ASP.NET Core MVC includes built-in attribute classes in the System.ComponentModel.DataAnnotations namespace that can be used to define metadata for ASP.NET MVC and ASP.NET data controls. These attributes are used to display appropriate validation messages to the users.
Overall, validation is an important part of the user interface of a web application as it helps ensure the accuracy, completeness, and security of user input.
