c#异步async和await

新建一个wpf程序,界面上拖入进度条和按钮控件设置进度条属性为IsIndeterminate="True"

wpf的XAML代码:

 <Button Content="Button" HorizontalAlignment="Left" Margin="274,264,0,0" VerticalAlignment="Top" Width="173" Height="60" Click="Button_Click"/>
 <ProgressBar HorizontalAlignment="Left" Height="142" Margin="96,80,0,0" VerticalAlignment="Top" Width="562" IsIndeterminate="True" />

　按钮点击事件:

 private void Button_Click(object sender, RoutedEventArgs e)
 {
      WebClient ws = new WebClient();
      String result=ws.DownloadString("https://gitzab.com");
      MessageBox.Show(result);
 }

　运行程序点击按钮会发现进度条卡住、按钮、窗口均无法操作,这里必须等待下载好执行页面的html字符串才能继续对这个窗口的操作,显然这里用同步的方式去做这种耗时的操作不适合.

 异步实现:

 private void Button_Click(object sender, RoutedEventArgs e)
 {
        //异步的方式
        WebClient wc = new WebClient();
        var downloadThread = new Thread(() =>
        {
            var result = wc.DownloadString("https://www.bilibili.com/");
            MessageBox.Show(result);
        });
        downloadThread.Start();
}

　我们希望下载网页字符串完成前不允许再次点击button按钮，修改代码如下:

 

 private void Button_Click(object sender, RoutedEventArgs e)
        {
            DownLoadButton.IsEnabled = false;
            WebClient wc = new WebClient();
            var downloadThread = new Thread(() =>
            {
                var result = wc.DownloadString("https://www.bilibili.com/");
                MessageBox.Show(result);
                //由于DownLoadButton按钮是在主线程中执行的,新开的线程无法操作,所以这里回到主线程中对按钮进行操作
                Dispatcher.Invoke(()=> {
                    DownLoadButton.IsEnabled = true;
                });
            });

            downloadThread.Start();
        }

　　上述代码维护性，可读性都比较差,微软为了简化这个操作引入了async和await关键字来实现以上的操作,修改代码如下:

  private async void Button_Click(object sender, RoutedEventArgs e)
        {
            var result = await Save();
            MessageBox.Show(result);
        }

        public async Task<String> Save()
        {
            DownLoadButton.IsEnabled = false;
            var wc = new WebClient();
            var result = await wc.DownloadStringTaskAsync("https://www.163.com/");
            await Task.Run(()=> 
            {
                File.WriteAllText("d:/result.txt",result);
            });
            DownLoadButton.IsEnabled = true;

            return "Successfully saved the content";
        }