gistfile1.cs

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void StartButtonClick(object sender, RoutedEventArgs e)
    {
        this.Update();
    }

    private void StopButtonClick(object sender, RoutedEventArgs e)
    {
        cts.Cancel();
    }

    CancellationTokenSource cts;

    async void Update()
    {
        try
        {
            this.StartButton.IsEnabled = false;
            this.StopButton.IsEnabled = true;
            cts = new CancellationTokenSource();
            this.output.ItemsSource = null;
            this.output.ItemsSource = await GetLinksFromMsdnAsync();
        }
        catch (TaskCanceledException)
        {
            MessageBox.Show("Cancelled!");
        }
        finally
        {
            this.StartButton.IsEnabled = true;
            this.StopButton.IsEnabled = false;
        }
    }

    async Task<IEnumerable<string>> GetLinksFromMsdnAsync()
    {
        var response = await new WebClient()
            .DownloadStringTaskAsync(
                new Uri("http://msdn.microsoft.com/pt-br/"),
                cts.Token
            );

        var regex = "href\\s*=\\s*(?:\"(?<1>[^\"]*)\"|(?<1>\\S+))";

        return Regex.Matches(response, regex)
                .OfType<Match>()
                .Select(m => m.Groups[1].Value);
    }
}