MainWindow.xaml.cs

using System.Diagnostics;
using System.Net.Http;
using System.Windows;

using Microsoft.Extensions.DependencyInjection;

namespace AsyncAwait;

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

    public MainWindow()
    {
        InitializeComponent();

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

        okButton.Click += async (s, e) =>
        {
            using (HttpClient w = this.clientFactory.CreateClient())
            {
                infoTextBlock.Text = await w.GetStringAsync(uriTextBox.Text);
            }
        };
    }

    private void fetchHeadersButton_Click(object sender, RoutedEventArgs e)
    {
        FetchAndShowHeaders("https://endjin.com/", this.clientFactory);
        Debug.WriteLine("Method returned");
    }

    // Note: as you'll see later, async methods usually should not be void
    private async void FetchAndShowHeaders(string url, IHttpClientFactory cf)
    {
        using (HttpClient w = cf.CreateClient())
        {
            var req = new HttpRequestMessage(HttpMethod.Head, url);
            HttpResponseMessage response =
                await w.SendAsync(req, HttpCompletionOption.ResponseHeadersRead);

            headerListTextBox.Text = response.Headers.ToString();
        }
    }

    private void OldSchoolFetchHeaders(string url, IHttpClientFactory cf)
    {
        HttpClient w = cf.CreateClient();
        var req = new HttpRequestMessage(HttpMethod.Head, url);

        var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
        w.SendAsync(req, HttpCompletionOption.ResponseHeadersRead)
            .ContinueWith(sendTask =>
            {
                try
                {
                    HttpResponseMessage response = sendTask.Result;
                    headerListTextBox.Text = response.Headers.ToString();
                }
                finally
                {
                    w.Dispose();
                }
            },
            uiScheduler);
    }
}