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c# - Wep API 无法从 HttpClient 调用返回数据

问题描述

我无法弄清楚为什么会发生以下问题：

我创建了一个简单的 Web api 控制器（.Net Framework 4.6.1）

public class TestController : ApiController
{
    // GET api/<controller>
    public async Task<string> Get()
    {
        var url = "http://api.plos.org/search?q=title:DNA";
        using (var httpClient = new HttpClient())
        {
            using (var response = await httpClient.GetAsync(url))
            {
                using (var content = response.Content)
                {
                    var result = await content.ReadAsStringAsync();
                    return result;

                }
            }
        }
    }
}

当我运行上面的代码时，大约 20 秒后出现以下错误。

{
    "Message": "An error has occurred.",
    "ExceptionMessage": "An error occurred while sending the request.",
    "ExceptionType": "System.Net.Http.HttpRequestException",
    "StackTrace": "   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()\r\n   at restapitest2.Controllers.TestController.<Get>d__0.MoveNext() in C:\\restapitest2\\restapitest2\\Controllers\\TestController.cs:line 19\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Threading.Tasks.TaskHelpersExtensions.<CastToObject>d__1`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__5.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__15.MoveNext()",
    "InnerException": {
        "Message": "An error has occurred.",
        "ExceptionMessage": "Unable to connect to the remote server",
        "ExceptionType": "System.Net.WebException",
        "StackTrace": "   at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)\r\n   at System.Net.Http.HttpClientHandler.GetResponseCallback(IAsyncResult ar)",
        "InnerException": {
            "Message": "An error has occurred.",
            "ExceptionMessage": "A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 216.74.38.80:80",
            "ExceptionType": "System.Net.Sockets.SocketException",
            "StackTrace": "   at System.Net.Sockets.Socket.InternalEndConnect(IAsyncResult asyncResult)\r\n   at System.Net.Sockets.Socket.EndConnect(IAsyncResult asyncResult)\r\n   at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure, Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState state, IAsyncResult asyncResult, Exception& exception)"
        }
    }
}

当我创建一个 dotnet core web api 控制器时，相同的代码可以正常工作。

这就是点网核心代码的样子。

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // GET api/values
    [HttpGet]
    public async Task<ActionResult<string>> Get()
    {
        var url = "http://api.plos.org/search?q=title:DNA";
        using (var httpClient = new HttpClient())
        {
            using (var response = await httpClient.GetAsync(url))
            {
                using (var content = response.Content)
                {
                    var result = await content.ReadAsStringAsync();
                    return result;

                }
            }
        }
    }
}

奇怪的是，如果我使用内部 URL，两个 api 解决方案都会返回数据。

任何帮助，将不胜感激

标签： c#.netasp.net-web-api

解决方案

你确定你不是在代理后面吗？

你确定你的 DNS 服务器可以查到api.plos.org吗？

更深入地检查您的异常，检查其内部异常将帮助您了解是 DNS 问题还是代理问题。

如果您使用代理，请不要忘记为 HttpClient 设置代理。

更新1：

连接尝试失败，因为连接的一方在一段时间后没有正确响应，或者连接失败，因为连接的主机没有响应 216.74.38.80:80

每次在我的机器上获得代理时，我都会遇到这个错误。既然您说“如果您运行 fiddler 一切正常”，我假设您需要从 windows 中删除代理设置Lan Settings。

  1. 在run类型中inetcpl.cpl并按回车。
  2. 转到Connections选项卡。
  3. 在对话框的底部，有一个Lan Settings按钮。点击它。
  4. 在 LAN 设置对话框中，确保您已取消选中所有复选框，尤其是use proxy for your lan。

题外话推荐：

HttpClient 旨在被实例化一次并在应用程序的整个生命周期中重复使用。为每个请求实例化一个 HttpClient 类将耗尽重负载下可用的套接字数量。这将导致 SocketException 错误。下面是一个正确使用 HttpClient 的例子。

public class GoodController : ApiController
{
    // OK
    private static readonly HttpClient HttpClient;

    static GoodController()
    {
        HttpClient = new HttpClient();
    }
}

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