You can add input validation to Web Forms pages by using validation controls.
To make sure that all the required parameters exist in a request, use the "RequiredFieldValidator" validation control. This control ensures that the user does not skip an entry in the web form.
To make sure user input contains only valid values, you can use one of the following validation controls:
[1] "RangeValidator": checks boundary value in between specified lower and upper boundaries. You can check ranges within pairs of numbers, alphabetic characters, and dates.
[2] "RegularExpressionValidator": checks that the entry matches a pattern defined by a regular expression. Validation allows to check for predictable sequences of characters, such as those in social security numbers, e-mail addresses, telephone numbers, postal codes, and so on.
<form id="form1" runat="server"> Your name:<br /> <asp:TextBox runat="server" id="txtName" /> <asp:RequiredFieldValidator runat="server" id="reqName" controltovalidate="txtName" errormessage="Please enter your name!" /> <br /><br /> <asp:Button runat="server" id="btnSubmitForm" text="Ok" /></form>