c# ComboBox SelectedindexChanged (combobox filled withSQL)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
namespace Combobox
{
    public partial class Form1 : Form
    {
        string ConnectionString = System.Configuration.ConfigurationSettings.AppSettings["dsn"];
        OleDbCommand com ,com1;
        string str;
       

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            comboBox1.Items.Add("Choose CourseID");
            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Courses";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                comboBox1.Items.Add(reader["CourseID"]); 
            }
            reader.Close();
            con.Close();


            con.Open();
            str = "select * from Students";
            com1 = new OleDbCommand(str, con);
            OleDbDataReader reader1 = com1.ExecuteReader();
            while (reader1.Read())
            {
                comboBox1.Items.Add(reader1["StudentID"]);
            }
            reader1.Close();
            con.Close();
            
        }

        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

            OleDbConnection con = new OleDbConnection(ConnectionString);
            con.Open();
            str = "select * from Courses where CourseID='" + comboBox1.Text.Trim() + "'";
            com = new OleDbCommand(str, con);
            OleDbDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                label2.Text = reader["CourseTitle"].ToString();
               
            }
            reader.Close();
            con.Close();
           

            con.Open();
            str = "select * from Students where StudentID='" + comboBox1.Text.Trim() + "'";
            com1 = new OleDbCommand(str, con);
            OleDbDataReader reader1 = com1.ExecuteReader();
            while (reader1.Read())
            {
                label2.Text = reader1["FirstName"].ToString();
              
            }
            reader.Close();
            con.Close(); 
        }
    }
}