调用 httpClient.GetAsync() 时，发送任何数据失败，有时捕获到 TaskCanceledException [重复]

Question

我开发了 C# .net 4.6.1 应用程序（这是Windows Service Application）。在应用程序中，我使用 HttpClient 与我们的后端 API 进行通信。随着时间的推移，应用程序不会向我们的后端发送请求（TaskCanceledException 被捕获）。

这里是这个异常的堆栈跟踪

System.Threading.Tasks.TaskCanceledException · 任务被取消。

System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
TestHttpClient+<GetRequestAsync>d__20.MoveNext()

这个问题有两个原因解决

1. 重启应用后
2. 重复尝试相同的请求时

我使用Fiddler 调查了这个问题，当异常发生且处于正常状态时

1. In normal mode fiddler shows 2 requests

• Result=200，Protocol=HTTP，Host = Tunnel to，Url = api.example.com:443

在 SyntaxView 选项卡中：找到了与 SSLv3 兼容的 ClientHello 握手。 Fiddler 提取了以下参数。

• 结果 = 200，协议 = HTTPS，主机 = api.example.com，网址 = /test

1. In failed mode Fiddler shows only 1 requst

• Result=200，Protocol=HTTP，Host = Tunnel to，Url = api.example.com:443，

在 SyntaxView 选项卡中： 客户收到通知后 建立CONNECT，发送数据失败

这是我们代码的 sn-p。

class Program
{
  static void Main(string[] args)
  {
    // I Tried also without  Task.Run() i.e. MyClass myClass = GetAsync().Result; 
    MyClass myClass = Task.Run(() => GetAsync().Result).Result ;
  }
        
  private static async Task<MyClass> GetAsync()
  {
    // I Tried also without .ConfigureAwait(false)
     MyClassResponse myClassResponse = await TestHttpClient.GetMyClassResponse().ConfigureAwait(false);
    return MyClass.Create(myClassResponse);
  }
}

public static class TestHttpClient
{
   private static HttpClient _httpClient;
   
   public static void Init()
   {
      ServicePointManager.SecurityProtocol |= (SecurityProtocolType.Ssl3 | 
                SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | 
                                             SecurityProtocolType.Tls);
      ServicePointManager.DefaultConnectionLimit = 10;
     _httpClient = CreateHttpClient();
   }

   private static HttpClient CreateHttpClient()
   {
      HttpClient client = new HttpClient();
      client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("token .....");
      client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
      client.Timeout = TimeSpan.FromMilliseconds(10000);
      return client;
    }

   public static async Task<MyClassResponse> GetMyClassResponse()
   {
      HttpResponseMessage response = await GetRequestAsync("https://api.example.com/test");
      return await ParseToMyClassResponse<MyClassResponse>(response);
   }
 
    private static async Task<HttpResponseMessage> GetRequestAsync(string url)
   {
        try
        {
           return await _httpClient.GetAsync(new Uri(url));
         }
         catch (TaskCanceledException)
         {
          return new HttpResponseMessage(HttpStatusCode.RequestTimeout);
         }
    }

    private static async Task<T> ParseToMyClassResponse<T>(HttpResponseMessage response) where T : MyClassResponse, new()
        {
            T myClassResponse;
            try
            {
                string content = await response.Content.ReadAsStringAsync();
                response.EnsureSuccessStatusCode();
                myClassResponse = JsonConvert.DeserializeObject<T>(content);
                           }
            catch (Exception ex)
            {
                myClassResponse = new T();
            }

            response.Dispose();
            return myClassResponse;
        }
}

我做错了什么？

为什么Fiddler 显示"After the client received notice of the established CONNECT, it failed to send any data" 消息。

Answer 1

1. 首先替换这个

static void Main(string[] args)
{
    // I Tried also without  Task.Run() i.e. MyClass myClass = GetAsync().Result; 
    MyClass myClass = Task.Run(() => GetAsync().Result).Result ;
}

与 whis

static async Task Main(string[] args)
{
    MyClass myClass = await GetAsync();
    
    Console.WriteLine("All done.");
    Console.ReadKey();
}

1. HttpResponseMessage 是IDisposable，请妥善处理

public static async Task<MyClassResponse> GetMyClassResponse()
{
    using (HttpResponseMessage response = await GetRequestAsync("https://api.example.com/test"))
    {
        return await ParseToMyClassResponse<MyClassResponse>(response);
    }
}

并从ParseToMyClassResponse 中删除response.Dispose()

1. 为了进行额外的测试，我重写了代码以使其更简单。

public class Program
{
    private readonly static HttpClient client = new HttpClient();

    static async Task Main(string[] args)
    {
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("token .....");
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
        client.Timeout = TimeSpan.FromMilliseconds(10000);
        ServicePointManager.DefaultConnectionLimit = 10;

        try
        {
            MyClassResponse myClassResponse = await GetAPIResponseAsync("https://api.example.com/test");
            MyClass myClass = MyClass.Create(myClassResponse);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
            Console.WriteLine();
            Console.WriteLine(ex.StackTrace);
        }

        Console.WriteLine("All done.");
        Console.ReadKey();
    }

    private static async Task<T> GetAPIResponseAsync<T>(string url)
    {
        using (HttpResponseMessage response = await client.GetAsync(url, HttpCompletionOption.ResponseHeadersRead))
        {
            response.EnsureSuccessStatusCode();
            string content = await response.Content.ReadAsStringAsync();
            return JsonConvert.DeserializeObject<T>(content);
        }
    }
}

测试这个版本。

此外，不建议在代码中更改SecurityProtocol，因为这会使应用程序易受攻击，请改用默认值。如果您想更改 TLS 版本进行测试，edit Windows Registry。

---

更新

我看过源代码。

这是一种糟糕的并发编程。您在任务中运行异步函数并阻塞正在执行它的任务并同时阻塞当前线程。

Task<PassResponse> task = Task.Run(() => GetPass(scannedValue).Result);
PassResponse passResponse = task.Result;

这样你可能会得到同样的结果

PassResponse passResponse = GetPass(scannedValue).Result;

---

再试一次

让我们改变这一行以避免可能的死锁

return await _httpClient.GetAsync(new Uri(url));

到

return await _httpClient.GetAsync(new Uri(url)).ConfigureAwait(false);

还有这个

string content = await response.Content.ReadAsStringAsync();

到

string content = await response.Content.ReadAsStringAsync().ConfigureAwait(false);