Async.cs

class Async
    {
        static IEnumerable<Uri> uris = new List<Uri>()
        {
            new Uri("http://www.google.com"),
            new Uri("http://www.bing.com"),
            new Uri("http://www.yahoo.com")
        };

        public static void DownloadStringAsync()
        {
            var client = new WebClient();

            client.DownloadStringCompleted += (sender, e) =>
            {
                if (e.Error == null)
                {
                    Console.WriteLine(e.Result.Substring(0, 20));
                    var client2 = new WebClient();
                    client2.DownloadStringCompleted += (sender2, e2) =>
                        {
                            if (e2.Error == null)
                            {
                                Console.WriteLine(e2.Result.Substring(0, 20));
                                var client3 = new WebClient();
                                client3.DownloadStringCompleted += (sender3, e3) =>
                                {
                                    if (e3.Error == null) Console.WriteLine(e3.Result.Substring(0, 20));
                                    else Console.WriteLine(e3.Error.Message);
                                };
                                client3.DownloadStringAsync(new Uri("http://www.yahoo.com"));
                            }
                            else Console.WriteLine(e2.Error.Message);
                        };
                    client2.DownloadStringAsync(new Uri("http://www.bing.com"));
                }
                else Console.WriteLine(e.Error.Message);
            };

            client.DownloadStringAsync(new Uri("http://www.google.com"));
        }

        public static void DownloadStringAsyncParallel()
        {
            foreach (var uri in uris)
          {
                var client = new WebClient();

                client.DownloadStringCompleted += (sender, e) => 
                    {
                        if (e.Error == null) Console.WriteLine(e.Result.Substring(0, 10));
                        else Console.WriteLine(e.Error.Message);
                    };

                client.DownloadStringAsync(uri);
	        }
        }

        public static async void DownloadStringTaskAsync()
        {
            var client = new WebClient();

            try
            {
                Console.WriteLine(await client.DownloadStringTaskAsync("http://www.google.com"));
                Console.WriteLine(await client.DownloadStringTaskAsync("http://www.bing.com"));
                Console.WriteLine(await client.DownloadStringTaskAsync("http://www.yahoo.com"));
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }

        public static async void DownloadStringTaskAsyncParallel()
        {
            var client = new WebClient();

            try
            {
                var htmls = await Task.WhenAll(uris.Select(uri => new WebClient().DownloadStringTaskAsync(uri)));

                foreach (var html in htmls)
                    Console.WriteLine(html.Substring(0, 30));
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
        }
    }