private void EnterTextBox_KeyDown(object sender, KeyEventArgs e)
        {
            switch (e.KeyCode)
            {
            case Keys.Enter:
                Button1_Click(this, new EventArgs());
                break;

            case Keys.Up:
                e.Handled = true;
                string prevCommand = commands.GetPrevious();
                textBox1.Text           = prevCommand;
                textBox1.SelectionStart = prevCommand.Length;
                break;

            case Keys.Down:
                e.Handled = true;
                string nextCommand = commands.GetNext();
                textBox1.Text           = nextCommand;
                textBox1.SelectionStart = nextCommand.Length;
                break;
            }
        }