Using dropdown selection textbox is visible

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.IO;



namespace WindowsFormsApplication1
{
   
    
    public partial class Form1 : Form
    {
        BindingList<string> course = new BindingList<string>() { "AFF", "PST", "Please add to new course" };
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            DropDownList1.DataSource = course;
            DropDownList1.DisplayMember = "course";
            DropDownList1.ValueMember = "course";
            textBox1.Visible = false;
            textBox1.Text = "Please add to new course";
        }

        private void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (DropDownList1.SelectedItem.ToString() == "Please add to new course")
            {
                textBox1.Visible = true;
            }
            else
            {
                textBox1.Visible = false;
            }
        }

        private void textBox1_Leave(object sender, EventArgs e)
        {
           
        }

        private void textBox1_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Enter && !course.Contains(textBox1.Text) && !string.IsNullOrEmpty(textBox1.Text))
            {
                course.Add(textBox1.Text);
                DropDownList1.SelectedItem = textBox1.Text;
            }
        } 

    }
}