如何异步使用HttpWebRequest(.NET)？

Question

如何异步使用HttpWebRequest(.NET,C#)？

Answer 1

使用 HttpWebRequest.BeginGetResponse()

HttpWebRequest webRequest;

void StartWebRequest()
{
    webRequest.BeginGetResponse(new AsyncCallback(FinishWebRequest), null);
}

void FinishWebRequest(IAsyncResult result)
{
    webRequest.EndGetResponse(result);
}

异步操作完成时调用回调函数.您至少需要EndGetResponse()从此功能调用.

Answer 2

考虑到答案:

HttpWebRequest webRequest;

void StartWebRequest()
{
    webRequest.BeginGetResponse(new AsyncCallback(FinishWebRequest), null);
}

void FinishWebRequest(IAsyncResult result)
{
    webRequest.EndGetResponse(result);
}

您可以发送请求指针或任何其他对象,如下所示:

void StartWebRequest()
{
    HttpWebRequest webRequest = ...;
    webRequest.BeginGetResponse(new AsyncCallback(FinishWebRequest), webRequest);
}

void FinishWebRequest(IAsyncResult result)
{
    HttpWebResponse response = (result.AsyncState as HttpWebRequest).EndGetResponse(result) as HttpWebResponse;
}

问候

Answer 3

到目前为止,每个人都错了,因为BeginGetResponse()有些人在当前的线程上工作.从文档:

在此方法变为异步之前,BeginGetResponse方法需要完成一些同步设置任务(例如,DNS解析,代理检测和TCP套接字连接).因此,永远不应在用户界面(UI)线程上调用此方法,因为在抛出错误的异常之前,可能需要相当长的时间(最多几分钟,具体取决于网络设置)才能完成初始同步设置任务.方法成功.

所以要做到这一点:

void DoWithResponse(HttpWebRequest request, Action<HttpWebResponse> responseAction)
{
    Action wrapperAction = () =>
    {
        request.BeginGetResponse(new AsyncCallback((iar) =>
        {
            var response = (HttpWebResponse)((HttpWebRequest)iar.AsyncState).EndGetResponse(iar);
            responseAction(response);
        }), request);
    };
    wrapperAction.BeginInvoke(new AsyncCallback((iar) =>
    {
        var action = (Action)iar.AsyncState;
        action.EndInvoke(iar);
    }), wrapperAction);
}

然后,您可以通过响应执行所需操作.例如:

HttpWebRequest request;
// init your request...then:
DoWithResponse(request, (response) => {
    var body = new StreamReader(response.GetResponseStream()).ReadToEnd();
    Console.Write(body);
});

Answer 4

到目前为止,最简单的方法是使用TaskFactory.FromAsync从TPL.当与新的async/await关键字一起使用时,它实际上是几行代码:

var request = WebRequest.Create("http://www.stackoverflow.com");
var response = (HttpWebResponse) await Task.Factory
    .FromAsync<WebResponse>(request.BeginGetResponse,
                            request.EndGetResponse,
                            null);
Debug.Assert(response.StatusCode == HttpStatusCode.OK);

如果您不能使用C#5编译器,则可以使用Task.ContinueWith方法完成上述操作:

Task.Factory.FromAsync<WebResponse>(request.BeginGetResponse,
                                    request.EndGetResponse,
                                    null)
    .ContinueWith(task =>
    {
        var response = (HttpWebResponse) task.Result;
        Debug.Assert(response.StatusCode == HttpStatusCode.OK);
    });

Answer 5

我最终使用了BackgroundWorker,它绝对是异步的,不像上面的一些解决方案,它处理返回GUI线程,它很容易理解.

它也很容易处理异常,因为它们最终出现在RunWorkerCompleted方法中,但请确保您阅读:BackgroundWorker中的未处理异常

我使用过WebClient但显然你可以使用HttpWebRequest.GetResponse.

var worker = new BackgroundWorker();

worker.DoWork += (sender, args) => {
    args.Result = new WebClient().DownloadString(settings.test_url);
};

worker.RunWorkerCompleted += (sender, e) => {
    if (e.Error != null) {
        connectivityLabel.Text = "Error: " + e.Error.Message;
    } else {
        connectivityLabel.Text = "Connectivity OK";
        Log.d("result:" + e.Result);
    }
};

connectivityLabel.Text = "Testing Connectivity";
worker.RunWorkerAsync();

Answer 6

自从发布了许多这些答案以来,.NET已经发生了变化,我想提供一个更新的答案.使用异步方法启动Task将在后台线程上运行的方法:

private async Task<String> MakeRequestAsync(String url)
{    
    String responseText = await Task.Run(() =>
    {
        try
        {
            HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
            WebResponse response = request.GetResponse();            
            Stream responseStream = response.GetResponseStream();
            return new StreamReader(responseStream).ReadToEnd();            
        }
        catch (Exception e)
        {
            Console.WriteLine("Error: " + e.Message);
        }
        return null;
    });

    return responseText;
}

要使用异步方法:

String response = await MakeRequestAsync("http://example.com/");

更新:

此解决方案不适用于WebRequest.GetResponseAsync()替代使用的UWP应用程序WebRequest.GetResponse(),并且它不会Dispose()在适当的地方调用方法.@dragansr有一个很好的替代解决方案来解决这些问题.

Answer 7

public static async Task<byte[]> GetBytesAsync(string url) {
    var request = (HttpWebRequest)WebRequest.Create(url);
    using (var response = await request.GetResponseAsync())
    using (var content = new MemoryStream())
    using (var responseStream = response.GetResponseStream()) {
        await responseStream.CopyToAsync(content);
        return content.ToArray();
    }
}

public static async Task<string> GetStringAsync(string url) {
    var bytes = await GetBytesAsync(url);
    return Encoding.UTF8.GetString(bytes, 0, bytes.Length);
}