.NET HttpClient 在使用 await 关键字时挂起？

Question

在考虑了这个有趣的答案HttpClient.GetAsync(...) never returns... 之后，我仍然遇到使用 await 时我的 HttpClient 没有返回的情况（下面的示例代码）。此外。我在 asp.net MVC5 控制器（UI 驱动）和 WebApi 中都使用了这个帮助程序。我能做什么：

1. 使用 await 代替（可怕的） .Result 并且仍然有这个函数返回？
2. 在 MVC 控制器和 WebApi 中重复使用相同的例程？

显然，我应该将 .Result 替换为 .ConfigureAwait(false) 但这似乎与上面引用的帖子中的“Task4”与等待 httpClient.GetAsync 一起工作的事实相矛盾。还是我需要单独的控制器和 WebApi 例程？

public static async Task<IEnumerable<TcMarketUserFullV1>> TcSearchMultiUsersAsync(string elasticQuery)
{
    if (string.IsNullOrEmpty(elasticQuery)) return null;

    IEnumerable<TcMarketUserFullV1> res = null;
    using (var hclient = new HttpClient())
    {
        hclient.BaseAddress = new Uri("https://addr.servicex.com");
        hclient.DefaultRequestHeaders.Accept.Clear();
        hclient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        hclient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",
            CloudConfigurationManager.GetSetting("jwt-bearer-token"));

        // Why does this never return when await is used?
        HttpResponseMessage response =  hclient.GetAsync("api/v2/users?q=" + elasticQuery + "&search_engine=v2").Result; 
        if (response.IsSuccessStatusCode)
        {
            var content = await response.Content.ReadAsStringAsync();
            res = JsonConvert.DeserializeObject<TcMarketUserFullV1[]>(content).AsEnumerable();
        }
        else{log.Warn("...");}
    } 
    return res;
} 

更新：我的调用链，从 Telerik Kendo Mvc.Grid DataBinding 调用开始如下：

[HttpPost]
public async Task<ActionResult> TopLicenseGrid_Read([DataSourceRequest]DataSourceRequest request)
{
    var res = await GetLicenseInfo();
    return Json(res.ToDataSourceResult(request)); // Kendo.Mvc.Extensions.DataSourceRequest
}

然后：

private async Task<IEnumerable<CsoPortalLicenseInfoModel>> GetLicenseInfo()
{
  ...
  // Never returns
  var qry = @"app_metadata.tc_app_user.country:""DE""";
  return await TcSearchMultiUsersAsync(qry);
}

然后，上面完整显示的例程，但现在没有 .Result：

public static async Task<IEnumerable<TcMarketUserFullV1>> TcSearchMultiUsersAsync(string elasticQuery)
{
    if (string.IsNullOrEmpty(elasticQuery)) return null;

    IEnumerable<TcMarketUserFullV1> res = null;
    using (var hclient = new HttpClient())
    {
        hclient.BaseAddress = new Uri("https://addr.servicex.com");
        hclient.DefaultRequestHeaders.Accept.Clear();
        hclient.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));

        hclient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",
            CloudConfigurationManager.GetSetting("jwt-bearer-token"));

        // This now returns fine
        HttpResponseMessage response =  hclient.GetAsync("api/v2/users?search_engine=v2&q=" + elasticQuery"); 
        if (response.IsSuccessStatusCode)
        {
            // This returns my results fine too
            var content = await response.Content.ReadAsStringAsync();
            // The following line never returns results. When debugging, everything flows perfectly until I reach the following line, which never 
            // returns and the debugger returns me immediately to the top level HttpPost with a result of null. 
            res = JsonConvert.DeserializeObject<TcMarketUserFullV1[]>(content).AsEnumerable();
        }
        else{log.Warn("...");}
    } 
    return res;
} 

Answer 1

你需要await 一切。你在其中一个上await 是不够的，你需要在所有这些上await：

这个：

HttpResponseMessage response = hclient.GetAsync(
                        "api/v2/users?q=" + elasticQuery + "&search_engine=v2").Result;

应该是：

HttpResponseMessage response = await hclient.GetAsync(
                                "api/v2/users?q=" + elasticQuery + "&search_engine=v2");

为了死锁，对.Result 进行一次阻塞调用就足够了。你需要"async all the way"。