Creating a Poll System Using ASP.NET 2.0 Callbacks

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;
using System.Data.SqlClient;

namespace WeeklyPollDemoWebApps
{
    public partial class TestingSessionErrors : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            GetObjectFromSession();
        }

        private string GetObjectFromSession()
        {
            if (Session["UserName"] == null)
                throw new ArgumentNullException("UserName is null","UserName is null. Your session has expired. <res> Please log on to the system to start a new session </res>");

            return Session["UserName"] as String; 

        }

        protected void Button2_Click(object sender, EventArgs e)
        {
            throw new InvalidOperationException("You can not allowed to perform this operation. <res> Please check with your system administrator for your rights </res>"); 
        }

        protected void Button3_Click(object sender, EventArgs e)
        {
            throw new DivideByZeroException("Divide by Zero Error. <res> You are trying to divide a number by zero </res>");
            
        }

        protected void Button4_Click(object sender, EventArgs e)
        {
            string connectionString = "Server=localhost;Database=BABA;Trusted_Connection=true";

            SqlConnection myConnection = null;
            try
            {
                myConnection = new SqlConnection(connectionString);
                myConnection.Open();
            }
            catch (Exception ex)
            {
                throw new ArgumentNullException("The database does not exist. <res> Please make sure the database you are trying to connect exists. </res>"); 
            }
            finally { myConnection.Close(); } 

        }
    }
}