C# 若我将listbox刷新方法移动到页面加载中，listboxselectedindex将停止工作

我正在使用ASP.NET，我有一个刷新按钮。一个列表框。两个文本框。在.cs方面，我有一个方法，它清除列表框并刷新列表框，如果我将其放入页面加载，listbox selectedindexchanged事件将停止工作。为什么会这样

using System;
using System.Collections.Generic;
using System.Data.SqlClient;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

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

    }

    SqlConnection cnn = new SqlConnection("Initial Catalog=bigum;Data Source=localhost;Integrated Security=SSPI;");

    protected void refresh()
    {

        ListBox1.Items.Clear();

        cnn.Open();
        SqlCommand cmd = new SqlCommand("SELECT OgrenciFirstName,OgrenciLastName FROM ogrenciler", cnn);
        SqlDataReader dr = cmd.ExecuteReader();
        if (dr.HasRows)
        {
            while (dr.Read())
            {
                ListBox1.Items.Add(dr.GetString(0) + " " + dr.GetString(1));
            }
        }

        cnn.Close();

    }


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


    protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
    {

        if (ListBox1.SelectedIndex > -1)
        {
            cnn.Open();
            SqlCommand cmd = new SqlCommand("SELECT OgrenciFirstName,OgrenciLastName FROM ogrenciler WHERE OgrenciID = @mynumber", cnn);
            cmd.Parameters.AddWithValue("@mynumber", (ListBox1.SelectedIndex + 1));
            SqlDataReader dr = cmd.ExecuteReader();
            if (dr.HasRows)
            {
                while (dr.Read())
                {
                    TextBox1.Text = dr.GetString(0);
                    TextBox2.Text = dr.GetString(1);
                }
            }

            cnn.Close();
        }

    }

---

page\加载在selectedIndex更改之前运行的代码，并导致dropdownlist selectedIndex=-1

protected void Page_Load(object sender, EventArgs e)
{
   if(!IsPostBack)
   {
      refresh();
   }
}