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; 
 }