How to make a textbox visible if last item in a combobox is selected c#

Question

I have combobox9,textBox4 and textBox15 in a winform. By default, textBox15 is hidden.

What i'm trying to do.

If textbox4 backcolor is red and the last item in combobox9 is selected, then show textbox15.

         if (comboBox9.SelectedIndex == comboBox9.Items.Count - 1  && textBox4.BackColor == Color.Red) ;

            {
                textBox15.Visible = true;

                textBox15.BackColor = Color.Red;
            }

            
            else
            {

                textBox15.Visible = false;
            }

No errors, it just doesn't work as expected. It keeps the textbox hidden even when the conditions are satisfied. Any leads?

Answer 1

You should put all your code in your SelectedIndexChanged event of your combobox

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if(comboBox1.SelectedIndex == comboBox1.Items.Count - 1)
    {
        label1.Visible = true;
    }
    else
    {
        label1.Visible = false;
    }
}