How to create and use class in asp.net 3.5

Refer a good article on CP which demonstrate how to insert and read data from a SQL Server:
Beginners guide to accessing SQL Server through C#[^]

A class is a construct that enables you to create your own custom types by grouping together variables of other types, methods and events.
It defines the data and behavior of a type.
Refer more details on following MSDN article:
Classes (C# Programming Guide)[^]

This is how you can create a class file and define connection string which can be accessed in you solution all over by creating object of this class file.

public class DBConnection
    {      
        public static string cnnStr;
        public SqlConnection cnTransact;
       
        public string GetConnectionString
        {
            get
            {
                return "ConnectionString"; //Connection strings name in Web.config file
            }
        }

        public DBConnection()
        {
            cnTransact = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
        }
    }

Web.config file:

<connectionstrings>
    <add name="ConnectionString" connectionstring="Server=SQLExpress;uid=abc;pwd=123;database=DBName" providername="System.Data.SqlClient" />
</connectionstrings>

Creating object of class file:

DBConnection DB = new DBConnection();

By this DB object you can access all the methods and properties in that class file in you solution. (Note that, DBConnection is class file name)

To open connection:

DB.cnTransact.Open();

To close connection:

DB.cnTransact.Close();

Refer more about classes:
C# Class[^]
Creating a Simple Class in C#[^]
Creating a Class Factory with C# and .NET[^]