标签不会在while循环中改变值

private void button1_Click(object sender, RoutedEventArgs e)
{
        int i = 0;

        while (i < 500)
        {
            label1.Content = i.ToString();
        //  System.Threading.Thread.Sleep(2000);
            ++i;
        }
 }
private void button1_Click(object sender, Route