DoWhile

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication27
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        int tal = 1;

        private void btnKnap_Click(object sender, EventArgs e)
        {
            do
            {
                lstUd.Items.Add(tal);
                tal = tal + 1;
            } while (tal < 10);

            while (tal >= 1)
            {
                lstUd.Items.Add(tal);
                tal = tal - 1;
            }
        }
    }
}