使用 .Result 的异步方法

Question

以下方法循环遍历 postdata 列表以对令牌发出多个请求，例如每个请求都使用特定的 clientID。我的问题与异步有关。我正在尝试对令牌进行异步调用。使用 .Result 是否必然使方法同步？

public async Task<string> ReturnDataFromUrl1(List<List<KeyValuePair<string, string>>> listOfPostData)
{        
   List<Task<string>> taskList = new List<Task<string>>();       
   string allTokens = "";
   List<Task<string>> downloadTasks = new List<Task<string>>();


   foreach (var postData in listOfPostData)
   {
       using (var client = new HttpClient())
       {
           client.BaseAddress = new Uri("http://localhost:23081");

           HttpContent httpContent = new FormUrlEncodedContent(postData);
           HttpResponseMessage response = client.PostAsync("/Token", httpContent).Result;

           var responsecode = (int)response.StatusCode;

           if (response.IsSuccessStatusCode)
           {
               var responseBodyAsText = response.Content.ReadAsStringAsync();

               taskList.Add(responseBodyAsText);
           }
       }
   }

    downloadTasks = taskList.ToList();            

    while (downloadTasks.Count > 0)
    {
        Task<string> firstFinishedTask = await Task.WhenAny(downloadTasks);

        downloadTasks.Remove(firstFinishedTask);

        // Await the completed task. 
        string content = await firstFinishedTask;

        allTokens = allTokens + content;
    }

    return allTokens;
}

Answer 1

Does the use of .Result necessarily make the method synchronous?

它将使 与 .Result 同步阻塞的部分 ，因为它同步等待 Task 的完成（如果您的代码不t 死锁，这在具有自定义 SynchronizationContext).

的环境中极有可能发生

此方法调用：

Task<string> firstFinishedTask = await Task.WhenAny(downloadTasks);

将是异步的，因为在读取第一个流并将其转换为字符串之前它将放弃控制权。如果您已经有一个标记为 async 的方法，那么也只需 await 那个部分：

HttpResponseMessage response = await client.PostAsync("/Token", httpContent);

旁注：

我想我会采取不同的方法来解决这个问题。通常，网络 IO 会为此方法消耗最多的时间。如果可能且没有限制，我会同时进行这些网络调用，然后在结果出现时对其进行处理：

public async Task<string> ReturnDataFromUrlAsync(
                                List<List<KeyValuePair<string, string>>> listOfPostData)
{
    var client = new HttpClient
    {
        BaseAddress = new Uri("http://localhost:23081")
    };

    var downloadTasks = listOfPostData.Select(postData =>
    {
        var content = new FormUrlEncodedContent(postData);
        return client.PostAsync("/Token", content);
    }).ToList();

    var tokenBuilder = new StringBuilder(downloadTasks.Count);
    while (downloadTasks.Count > 0)
    {
        var finishedTask = await Task.WhenAny(downloadTasks);
        downloadTasks.Remove(finishedTask);
        var response = await finishedTask;

        if (!response.IsSuccessStatusCode)
            continue;

        var token = await response.Content.ReadAsStringAsync();
        tokenBuilder.Append(token);
    }

    return tokenBuilder.ToString();
}

或者由于无论如何您都需要所有结果才能处理令牌，因此您可以使用 Task.WhenAll 等待所有结果完成：

public async Task<string> ReturnDataFromUrlAsync(
                                List<List<KeyValuePair<string, string>>> listOfPostData)
{
    var client = new HttpClient
    {
        BaseAddress = new Uri("http://localhost:23081")
    };

    var downloadTasks = listOfPostData.Select(postData =>
    {
        var content = new FormUrlEncodedContent(postData);
        return client.PostAsync("/Token", content);
    });

    HttpResponseMessage[] response = await Task.WhenAll(downloadTasks);

    var tokenBuilder = new StringBuilder(response.Length);
    foreach (var element in response.Where(message => message.IsSuccessStatusCode))
    {
        tokenBuilder.Append(await element.Content.ReadAsStringAsync());
    }
    return tokenBuilder.ToString();
}

使用 .Result 的异步方法

Asynchronous method using .Result

c#

asynchronous

httpclient

task-parallel-library

async-await