c# - HttpResponseMessage 没有响应

Question

我有如下服务的 MVC 项目:

namespace comp.Services
{
    public class CompService
    {
        public HttpClient client = new HttpClient();

        public CompService()
        {
            client.BaseAddress = new Uri("someapiurl");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));
        }

        protected async Task<string> GetProductAsync(string path)
        {
            var resp = "nothing here";
            HttpResponseMessage response = await client.GetAsync(path);
            if (response.IsSuccessStatusCode)
            {
                resp = await response.Content.ReadAsStringAsync();
            }
            return resp;
        }

        public string GetProduct(string path)
        {
            return GetProductAsync(path).GetAwaiter().GetResult();
        }
    }
}

和要查看的actionResult:

namespace comp.Controllers
{
    public class HomeController : Controller
    {
        public CompService compService;
        public HomeController()
        {
            compService = new CompService();
        }

        public ActionResult About()
        {
            var timeServer = compService.GetProduct("/api/time");
            ViewBag.timeServer = timeServer;
            return View();
        }
    }
}

在调试器中我遇到这一行:

HttpResponseMessage response = await client.GetAsync(path);

程序退出调试器，浏览器无响应。

在控制台应用程序中编写的相同代码可以工作。

在 VS 输出是响应成功的消息:

Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2018-02-18T13:48:31","tags":{"ai.internal.sdkVersion":"rddf:2.2.0-738","ai.internal.nodeName":"DESKTOP-xxxxx","ai.cloud.roleInstance":"DESKTOP-xxxxx"},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"/api/v1/time","id":"xxxxx=","data":"https://api.xxx.com/api/v1/time","duration":"00:00:01.3150000","resultCode":"200","success":true,"type":"Http","target":"xxx","properties":{"DeveloperMode":"true"}}}}

在浏览器控制台输出:

[14:51:33 GMT+0100 (Central European Standard Time)] Browser Link: Failed to send message to browser link server:
Error: SignalR: Connection must be started before data can be sent. Call .start() before .send()

感谢您的帮助。

Answer 1

你真的应该一直保持代码异步，而不是试图混契约(Contract)步和异步代码

namespace comp.Services {
    public class CompService {
        static HttpClient client = new HttpClient();

        static CompService() {
            client.BaseAddress = new Uri("someapiurl");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(
            new MediaTypeWithQualityHeaderValue("application/json"));
        }

        public async Task<string> GetProductAsync(string path) {
            var resp = string.Empty;
            using(var response = await client.GetAsync(path)) {
                if (response.IsSuccessStatusCode) {
                    resp = await response.Content.ReadAsStringAsync();
                }
            }
            return resp;
        }
    }
}

Controller Action 也应该是异步的

namespace comp.Controllers {
    public class HomeController : Controller {
        private CompService compService;

        public HomeController() {
            compService = new CompService();
        }

        public async Task<ActionResult> About() {
            var timeServer = await compService.GetProductAsync("api/time");
            ViewBag.timeServer = timeServer;
            return View();
        }
    }
}

也就是说，服务也应该被抽象

public interface ICompService {
    Task<string> GetProductAsync(string path)
}

public class CompService : ICompService {
    //...code removed for brevity
}

并注入(inject) Controller 而不是手动创建它。

public class HomeController : Controller {
    private ICompService compService;

    public HomeController(ICompService compService) {
        this.compService = compService;
    }

    public async Task<ActionResult> About() {
        var timeServer = await compService.GetProductAsync("api/time");
        ViewBag.timeServer = timeServer;
        return View();
    }
}

引用 Async/Await - Best Practices in Asynchronous Programming