how to use codes in c# classes?

You need this line at the top of the web page

asp.net Code:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="ColorChangeOnError.aspx.cs" Inherits="ColorChangeOnError" %>

CodeFile points to the *.cs you wish to use with the page and Inherits points to the method, see the following example

webpage

asp.net Code:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="ColorChangeOnError.aspx.cs" Inherits="ColorChangeOnError" %>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        
    <asp:Label
        id="lblFirstName"
        Text="First Name"
        AssociatedControlID="txtFirstName"
        Runat="server" />
    <br />    
    <asp:TextBox
        id="txtFirstName"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqFirstName"
        ControlToValidate="txtFirstName"
        Text="(Required)"
        EnableClientScript="false"
        Runat="server" />   
     
    <br /><br />
        
    <asp:Label
        id="lblLastName"
        Text="Last Name"
        AssociatedControlID="txtLastName"
        Runat="server" />
    <br />    
    <asp:TextBox
        id="txtLastname"
        Runat="server" />
    <asp:RequiredFieldValidator
        id="reqLastName"
        ControlToValidate="txtLastName"
        Text="(Required)" 
        EnableClientScript="false"
        Runat="server" />   
     
     <br /><br />
     
     <asp:Button
        id="btnSubmit"
        Text="Submit"
        Runat="server" OnClick="btnSubmit_Click" />   
 
    </div>
    </form>
</body>
</html>

cs file

C# Code:

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
 
public partial class ColorChangeOnError : System.Web.UI.Page
{
    protected void Page_PreRender()
    {
        foreach (BaseValidator valControl in Page.Validators)
        {
            WebControl ctrl = (WebControl)Page.FindControl(valControl.ControlToValidate);
            if (!valControl.IsValid)
                ctrl.BackColor = System.Drawing.Color.Yellow;
            else
                ctrl.BackColor = System.Drawing.Color.White;
        }
    }
    protected void btnSubmit_Click(object sender, EventArgs e)