Alternative for Thread.Sleep() to temporarily suspend program

Question

See the code at the bottom of this post. It's supposed to add "3" to the listbox, then "2" a second later, then "1" a second later and then run the main code of the program. However, once I execute the program it just stays blank 3 seconds long, after which all 3, 2, and 1 are shown, after which all the code directly starts. I want to visually see every number show up with a one second delay. How do I do this?

private void Main()
        {
            countdown();
            //Main Code
        }

private void countdown()
        {
            listBox1.Items.Clear();
            listBox1.Items.Add("3");
            System.Threading.Thread.Sleep(1000);
            listBox1.Items.Add("2");
            System.Threading.Thread.Sleep(1000);
            listBox1.Items.Add("1");
            System.Threading.Thread.Sleep(1000);
            listBox1.Items.Clear();
        }

Answer 1

async / await to the rescue:

private async void OnButtonClick(object sender, EventArgs e)
{
    listBox1.Items.Clear();
    listBox1.Items.Add("3");
    await Task.Delay(1000);

    listBox1.Items.Add("2");
    await Task.Delay(1000);

    listBox1.Items.Add("1");
    await Task.Delay(1000);

    listBox1.Items.Clear();
}