对于返回void类型的asyc的异步方法，如何修改，能使用await

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);
}

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;
}

}

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;
}