how to maintain session in asp 3.5 with c#

There are different ways. You can use form authentication , it's a way to do that.

<system.web>
  <authentication mode="Forms">
    <forms loginUrl="Login.aspx"
           protection="All"
           timeout="30"
           name=".ASPXAUTH" 
           path="/"
           requireSSL="false"
           slidingExpiration="true"
           defaultUrl="default.aspx"
           cookieless="UseDeviceProfile"
           enableCrossAppRedirects="false" />
  </authentication>

<authorization>  
    <allow users="admin" />
    <deny users="*" />
</authorization>
</system.web>
private void btnLogin_Click(object sender, System.EventArgs e)
{
 if (Validate(txtUsername.Text, txtPassword.Text))
 {
  FormsAuthentication.Initialize();
  String strRole = txtUsername.Text;  
  FormsAuthenticationTicket fat = new FormsAuthenticationTicket(1, 
   txtUsername.Text, DateTime.Now, 
   DateTime.Now.AddMinutes(30), false, strRole, 
   FormsAuthentication.FormsCookiePath);
  Response.Cookies.Add(new HttpCookie(FormsAuthentication.FormsCookieName, 
   FormsAuthentication.Encrypt(fat)));
  Response.Redirect(FormsAuthentication.GetRedirectUrl(txtUsername.Text, false));
 }
 else
  lblError.Visible = true;
}

private Boolean Validate(String strUsername, String strPassword)
{
    return ((strUsername == "admin") && (strPassword == "password"));
}
 

In logout page you can use the following code

private void Page_Load(object sender, System.EventArgs e)
{
 Session.Abandon();
 FormsAuthentication.SignOut();
}

remeber this kind of authentication works only user enables cookies