The remote server returned an error: (429) Too Many Requests

namespace AsyncDemoApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void executeSync_Click(object sender, RoutedEventArgs e)
        {
            var watch = System.Diagnostics.Stopwatch.StartNew();

            RunDownloadSync();

            watch.Stop();
            var elapsedMs = watch.ElapsedMilliseconds;

            resultsWindow.Text += $"Total execution time: {elapsedMs}";
        }

        private List<string> PrepData()
        {
            List<string> output = new List<string>();

            resultsWindow.Text = "";

            output.Add("https://www.yahoo.com");
            output.Add("https://www.cnn.com");
            output.Add("https://www.microsoft.com");
            output.Add("https://www.google.com");
            output.Add("https://www.stackoverflow.com");

            return output;
        }

        private void RunDownloadSync()
        {
            List<string> websites = PrepData();

            foreach(string site in websites)
            {
                WebsiteDataModel results = DownloadWebsite(site);
                ReportWebsiteInfo(results);

                System.Threading.Thread.Sleep(1000);
            }

        }
        
        
        
        
        private WebsiteDataModel DownloadWebsite(string websiteUrl)
        {
            WebsiteDataModel output = new WebsiteDataModel();
            WebClient client = new WebClient();

            output.WebsiteUrl = websiteUrl;
            output.WebsiteData = client.DownloadString(websiteUrl);

            return output;
        }

        private void ReportWebsiteInfo(WebsiteDataModel data)
        {
            resultsWindow.Text += $"{data.WebsiteUrl} downloaded: {data.WebsiteData.Length} characters long. {Environment.NewLine}";
        }
    }
}