How to set login authentication?

hi,

you can try like this:

in successfull login store the username in session. then every page in page load check the session is having value or not, if it having value then show the page if not redirect to loginpage:

ex:

in login page on successfull login  :

Session["UserID"]=textbox.Text;


in other pages in pageload:

protected void Page_Load(object sender, EventArgs e)
    {
       
            if (Session["UserID"] == null || (Session["UserID"] != null && Convert.ToString(Session["UserID"]) == String.Empty))
                Response.Redirect("login.aspx");
            else
            {
                //your code in page load
            }
      
    }