如何使HttpWebRequest异步

Question

我有这样的代码:

private async Task<string> Request(url)
    {
        Task<string> task = null;

        try
        {
            task = MakeAsyncRequest(url, "text/html");
            return await task;
        }

        catch
        {
            return null;
        }            
    } 

private async Task<string> MakeAsyncRequest(string url, string contentType)
    {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.ContentType = contentType;
        request.Method = WebRequestMethods.Http.Get;
        request.Timeout = 20000;
        request.Proxy = null;
        Task<WebResponse> task = Task.Factory.FromAsync(
        request.BeginGetResponse,
        asyncResult => request.EndGetResponse(asyncResult),
        (object)null);

            //issue here:
        return await task.ContinueWith(t => ReadStreamFromResponse(t.Result));
    }

private string ReadStreamFromResponse(WebResponse response)
    {
        using (Stream responseStream = response.GetResponseStream())
        using (StreamReader sr = new StreamReader(responseStream))
        {
            //Need to return this response 
            string strContent = sr.ReadToEnd();
            return strContent;
        }
    }

我Request(url)在foreach循环中打电话

foreach(var url in myUrlList)
{
  string body = Request(method).Result;
}

但由于某种原因,代码堆积在一起return await task.ContinueWith(t => ReadStreamFromResponse(t.Result));,只是冻结.

有没有更好的方法来做到这一点,或者有人可以解释发生了什么？我没有得到任何错误await......

Answer 1

该打电话给Result你的foreach循环导致死锁,因为我在我的博客解释.总之,await将捕获"上下文"(例如,UI上下文),并使用它来恢复该async方法.某些上下文(例如,UI上下文)仅允许上下文中的一个线程.因此,如果通过调用阻止该特殊线程(例如,UI线程)Result,则该async方法无法在该上下文中恢复执行.

所以,解决方案是改变你的foreach循环:

foreach(var url in myUrlList)
{
  string body = await ProcessAsync(method);
}

其他说明:

任务返回方法应以"异步"结束,以遵循TAP指南.

Task.Factory.FromAsync没必要; HttpWebRequest已经有了等待的方法.更好的选择是使用HttpClient.

我建议你不要使用Task.ContinueWith(或Task.Result,或Task.Wait); 使用await来代替.

通过这些简化:

private async Task<string> MakeAsyncRequestAsync(string url, string contentType)
{
  HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
  request.ContentType = contentType;
  request.Method = WebRequestMethods.Http.Get;
  request.Timeout = 20000;
  request.Proxy = null;
  WebResponse response = await request.GetResponseAsync();
  return ReadStreamFromResponse(response);
}

如果更改HttpWebRequest为,则可以进一步简化此代码HttpClient.