49

我想向服务器发送请求并处理返回值：

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    string result = string.Empty;
    responseTask.ContinueWith(x => result = Print(x));
    responseTask.Wait(); // it doesn't wait for the completion of the response task
    return result;
}

private static string Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    task.ContinueWith(t =>
    {
        Console.WriteLine("Result: " + t.Result);
        result = t.Result;
    });
    task.Wait();  // it does wait
    return result;
}

我使用Task正确吗？我不这么认为，因为该Send()方法string.Empty每次都返回，而Print返回正确的值。

我究竟做错了什么？如何从服务器获得正确的结果？

4

6 回答 6

43

您的 Print 方法可能需要等待继续完成（ContinueWith 返回一个您可以等待的任务）。否则，第二个 ReadAsStringAsync 完成，该方法返回（在继续分配结果之前）。您的发送方法中也存在同样的问题。两者都需要等待继续才能始终如一地获得您想要的结果。类似于下面

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    string result = string.Empty;
    Task continuation = responseTask.ContinueWith(x => result = Print(x));
    continuation.Wait();
    return result;
}

private static string Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    Task continuation = task.ContinueWith(t =>
    {
        Console.WriteLine("Result: " + t.Result);
        result = t.Result;
    });
    continuation.Wait();  
    return result;
}
于 2012-11-03T17:21:14.853 回答
8

它等待client.GetAsync("aaaaa");，但不等待result = Print(x)

尝试responseTask.ContinueWith(x => result = Print(x)).Wait()

- 编辑 - 

Task responseTask = Task.Run(() => { 
    Thread.Sleep(1000); 
    Console.WriteLine("In task"); 
});
responseTask.ContinueWith(t=>Console.WriteLine("In ContinueWith"));
responseTask.Wait();
Console.WriteLine("End");

上面的代码不保证输出：

In task
In ContinueWith
End

但这确实（参见newTask）

Task responseTask = Task.Run(() => { 
    Thread.Sleep(1000); 
    Console.WriteLine("In task"); 
});
Task newTask = responseTask.ContinueWith(t=>Console.WriteLine("In ContinueWith"));
newTask.Wait();
Console.WriteLine("End");
于 2012-11-03T17:05:53.803 回答
2

一个回答标题的干净示例

string output = "Error";
Task task = Task.Factory.StartNew(() =>
{
    System.Threading.Thread.Sleep(2000);
    output = "Complete";
});

task.Wait();
Console.WriteLine(output);
于 2018-10-06T07:44:31.960 回答
0

我是一个异步新手，所以我不能明确地告诉你这里发生了什么。我怀疑方法执行期望不匹配，即使您在方法内部使用任务。如果您将 Print 更改为返回 Task<string>，我认为您会得到预期的结果：

private static string Send(int id)
{
    Task<HttpResponseMessage> responseTask = client.GetAsync("aaaaa");
    Task<string> result;
    responseTask.ContinueWith(x => result = Print(x));
    result.Wait();
    responseTask.Wait(); // There's likely a better way to wait for both tasks without doing it in this awkward, consecutive way.
    return result.Result;
}

private static Task<string> Print(Task<HttpResponseMessage> httpTask)
{
    Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
    string result = string.Empty;
    task.ContinueWith(t =>
    {
        Console.WriteLine("Result: " + t.Result);
        result = t.Result;
    });
    return task;
}
于 2012-11-03T17:24:14.050 回答
0

在处理延续时，我发现将我编写 .ContinueWith 的地方视为执行立即继续到其后的语句的地方很有用，而不是它“内部”的语句。在这种情况下，很明显您将在 Send 中返回一个空字符串。如果您对响应的唯一处理是将其写入控制台，则在 Ito 的解决方案中不需要任何等待 - 控制台打印输出无需等待即可发生，但在这种情况下，发送和打印都应返回 void。在控制台应用程序中运行它，您将获得页面的打印输出。

IMO，等待和 Task.Result 调用（哪个块）有时是必要的，具体取决于您所需的控制流，但更多时候它们表明您没有真正正确使用异步功能。

namespace TaskTest
{
    class Program
    {
        static void Main(string[] args)
        {
            Send();
            Console.WriteLine("Press Enter to exit");
            Console.ReadLine();
        }

        private static void Send()
        {
            HttpClient client = new HttpClient();
            Task<HttpResponseMessage> responseTask = client.GetAsync("http://google.com");
            responseTask.ContinueWith(x => Print(x));
        }

        private static void Print(Task<HttpResponseMessage> httpTask)
        {
            Task<string> task = httpTask.Result.Content.ReadAsStringAsync();
            Task continuation = task.ContinueWith(t =>
            {
                Console.WriteLine("Result: " + t.Result);
            });
        }
    }
}
于 2012-11-05T01:53:07.500 回答
0 
async Task<int> AccessTheWebAsync()  
{   
    // You need to add a reference to System.Net.Http to declare client.  
    HttpClient client = new HttpClient();  

    // GetStringAsync returns a Task<string>. That means that when you await the  
    // task you'll get a string (urlContents).  
    Task<string> getStringTask = 

    client.GetStringAsync("http://msdn.microsoft.com");  

    // You can do work here that doesn't rely on the string from GetStringAsync.  
    DoIndependentWork();  

    // The await operator suspends AccessTheWebAsync.  
    //  - AccessTheWebAsync can't continue until getStringTask is complete.  
    //  - Meanwhile, control returns to the caller of AccessTheWebAsync.  
    //  - Control resumes here when getStringTask is complete.   
    //  - The await operator then retrieves the string result from 
    getStringTask.  
    string urlContents = await getStringTask;  

    // The return statement specifies an integer result.  
    // Any methods that are awaiting AccessTheWebenter code hereAsync retrieve the length 
    value.  
    return urlContents.Length;  
}
于 2018-04-24T16:53:02.803 回答