Understanding ASP.NET Validation Controls
Therefore, in ASP.NET you can use ASP.NET Validation Controls while creating form and specify what ASP.NET Validation Controls you want to use and to which server control you want bind this. ASP.NET Validation Controls are derived from a common base class and share a common set of properties and methods. You just have to drag and drop ASP.NET Validation Control in web form and write one line of code to describe its functionality.
The five types of ASP.NET Validation Controls in ASP.NET Framework include:
• RequiredField Validation control: prompts message if any input field is left blank. This validation control can also be used to prompt message to user if he or she has left any input field with its default value.