如何异步使用 HttpWebRequest （.NET）？

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。您至少需要从此函数调用 EndGetResponse（）。

考虑答案：

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

public void GetResponseAsync (HttpWebRequest request, Action<HttpWebResponse> gotResponse)
    {
        if (request != null) { 
            request.BeginGetRequestStream ((r) => {
                try { // there's a try/catch here because execution path is different from invokation one, exception here may cause a crash
                    HttpWebResponse response = request.EndGetResponse (r);
                    if (gotResponse != null) 
                        gotResponse (response);
                } catch (Exception x) {
                    Console.WriteLine ("Unable to get response for '" + request.RequestUri + "' Err: " + x);
                }
            }, null);
        } 
    }

到目前为止，每个人都错了，因为在当前线程上做了一些工作。从文档中：BeginGetResponse()

BeginGetResponse 方法需要一些同步安装任务才能 完成（DNS解析、代理检测、TCP套接字连接、 例如），在此方法变为异步之前。因此， 不应在用户界面 （UI） 线程上调用此方法 因为它可能需要相当长的时间（最多几分钟 取决于网络设置）以完成初始同步 在引发错误异常或方法之前设置任务 成功。

因此，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，您可以对响应执行所需的操作。例如：

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

我最终使用了 BackgroundWorker，与上述一些解决方案不同，它绝对是异步的，它为您处理返回到 GUI 线程，并且非常容易理解。

处理异常也非常容易，因为它们最终会出现在 RunWorkerCompleted 方法中，但请务必阅读以下内容：BackgroundWorker 中未处理的异常

我使用了 WebClient，但显然，如果您愿意，您可以使用 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();

到目前为止，最简单的方法是使用 TPL 中的 TaskFactory.FromAsync。当与新的 async/await 关键字结合使用时，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);

如果无法使用 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);
    });

自从发布其中许多答案以来，.NET 发生了变化，我想提供一个更新的答案。使用异步方法启动将在后台线程上运行的：Task

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 async 方法，请执行以下操作：

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

更新：

此解决方案不适用于使用 代替 的 UWP 应用，并且不会在适当的情况下调用这些方法。@dragansr 有一个很好的替代解决方案来解决这些问题。WebRequest.GetResponseAsync()WebRequest.GetResponse()Dispose()

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);
}

跟进@Isak的回答，很不错。尽管如此，它最大的缺陷是，只有当响应的状态为 200-299 时，它才会调用 responseAction。解决此问题的最佳方法是：

private void DoWithResponseAsync(HttpWebRequest request, Action<HttpWebResponse> responseAction)
{
    Action wrapperAction = () =>
    {
        request.BeginGetResponse(new AsyncCallback((iar) =>
        {
            HttpWebResponse response;
            try
            {
                response = (HttpWebResponse)((HttpWebRequest)iar.AsyncState).EndGetResponse(iar);
            }
            catch (WebException ex)
            {
                // It needs to be done like this in order to read responses with error status:
                response = ex.Response as HttpWebResponse;
            }
            responseAction(response);
        }), request);
    };
    wrapperAction.BeginInvoke(new AsyncCallback((iar) =>
    {
        var action = (Action)iar.AsyncState;
        action.EndInvoke(iar);
    }), wrapperAction);
}

然后如下@Isak：

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

我一直在将其用于异步 UWR，希望它对某人有所帮助

    string uri = "http://some.place.online";

    using (UnityWebRequest uwr = UnityWebRequest.Get(uri))
    {
        var asyncOp = uwr.SendWebRequest();
        while (asyncOp.isDone == false) await Task.Delay(1000 / 30); // 30 hertz

        if(uwr.result == UnityWebRequest.Result.Success) return uwr.downloadHandler.text;
        Debug.LogError(uwr.error);
    }