LoopWindow.xaml.cs

using System.IO;
using System.Net.Http;
using System.Windows;

using Microsoft.Extensions.DependencyInjection;

namespace AsyncAwait;

/// <summary>
/// Interaction logic for LoopWindow.xaml
/// </summary>
public partial class LoopWindow : Window
{
    private readonly IHttpClientFactory clientFactory;

    public LoopWindow()
    {
        InitializeComponent();

        var services = new ServiceCollection();
        services.AddHttpClient();
        this.clientFactory = services.BuildServiceProvider().GetRequiredService<IHttpClientFactory>();
    }

    private void fetchButton_Click(object sender, RoutedEventArgs e)
    {
        FetchAndShowBody(urlTextBox.Text, this.clientFactory);
    }

    // Example 11 shows this alternative
#if false
    private async Task FetchAndShowBody(string url, IHttpClientFactory cf)
#endif
    private async void FetchAndShowBody(string url, IHttpClientFactory cf)
    {
        using (HttpClient w = cf.CreateClient())
        {
            Stream body = await w.GetStreamAsync(url);
            using (var bodyTextReader = new StreamReader(body))
            {
                while (!bodyTextReader.EndOfStream)
                {
                    string? line = await bodyTextReader.ReadLineAsync();
                    bodyTextBox.AppendText(line);
                    bodyTextBox.AppendText(Environment.NewLine);
                    await Task.Delay(TimeSpan.FromMilliseconds(10));
                }
            }
        }
    }

    private void IncompleteOldSchoolFetchAndShowBody(
        string url, IHttpClientFactory cf)
    {
        HttpClient w = cf.CreateClient();
        var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
        w.GetStreamAsync(url).ContinueWith(getStreamTask =>
        {
            Stream body = getStreamTask.Result;
            var bodyTextReader = new StreamReader(body);

            StartNextIteration();

            void StartNextIteration()
            {
                if (!bodyTextReader.EndOfStream)
                {
                    bodyTextReader.ReadLineAsync().ContinueWith(readLineTask =>
                    {
                        string? line = readLineTask.Result;

                        bodyTextBox.AppendText(line);
                        bodyTextBox.AppendText(Environment.NewLine);

                        Task.Delay(TimeSpan.FromMilliseconds(10))
                            .ContinueWith(
                                _ => StartNextIteration(), uiScheduler);
                    },
                    uiScheduler);
                }
            };
        },
        uiScheduler);
    }

    public static async Task<string?> GetServerHeaderAsync(
        string url, IHttpClientFactory cf)
    {
        using (HttpClient w = cf.CreateClient())
        {
            var request = new HttpRequestMessage(HttpMethod.Head, url);
            HttpResponseMessage response = await w.SendAsync(
                request, HttpCompletionOption.ResponseHeadersRead);

            string? result = null;
            IEnumerable<string>? values;
            if (response.Headers.TryGetValues("Server", out values))
            {
                result = values.FirstOrDefault();
            }
            return result;
        }
    }
}