对于返回void类型的asyc的异步方法，如何修改，能使用await

下面是使用WebClinet 获取百度首页的html代码，一般的写法如下：

 private void Button_Click(object sender, RoutedEventArgs e)
 {
     WebClient client = new System.Net.WebClient();
     client.OpenReadAsync(new Uri("http://www.baidu.com/"));
     client.OpenReadCompleted += c_OpenReadCompleted;
 }
 void c_OpenReadCompleted(object sender, System.Net.OpenReadCompletedEventArgs e)
 {
     StreamReader reader = new StreamReader(e.Result);
     string content = reader.ReadToEnd();
     MessageBox.Show(content);
 } 

接下来是如何使用await：

首先添加一个扩展类

 public static class WebClientExtend
 {
     public static Task<Stream> OpenReadAsync(this WebClient webClient, string url)
     {
         TaskCompletionSource<Stream> source = new TaskCompletionSource<Stream>();
         webClient.OpenReadCompleted += (sender, e) =>
         {
             if (e.Error != null)
             {
                 source.TrySetException(e.Error);
             }
             else
             {
                 source.SetResult(e.Result);
             }
         };
         webClient.OpenReadAsync(new Uri(url, UriKind.RelativeOrAbsolute));
         return source.Task;
     }

 }

接下来使用await

  private async void Button_Click(object sender, RoutedEventArgs e)
  {
      WebClient client = new System.Net.WebClient();
      Stream stream = await client.OpenReadAsync("http://www.baidu.com/");
      StreamReader reader = new StreamReader(stream);
      string content = reader.ReadToEnd();
      MessageBox.Show(content);
      //client.OpenReadCompleted += c_OpenReadCompleted;
  }