如何等待任務在 C# 中完成？

Question

我想向服務器發送請求並處理返回值：

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;
}

我是否正確使用了Task ？ 我不這么認為，因為Send()方法每次都返回string.Empty ，而Print返回正確的值。

我究竟做錯了什么？ 如何從服務器獲得正確的結果？

Answer 1

您的 Print 方法可能需要等待繼續完成（ContinueWith 返回一個您可以等待的任務）。 否則，第二個 ReadAsStringAsync 完成，該方法返回（在延續中分配結果之前）。 您的發送方法中存在同樣的問題。 兩者都需要等待繼續以始終如一地獲得您想要的結果。 類似於下面

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;
}

Answer 2

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

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

- 編輯 -

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

上面的代碼不保證輸出：

In task
In ContinueWith
End

但這確實（見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");

Answer 3

我是一個異步新手，所以我不能確切地告訴你這里發生了什么。 我懷疑方法執行預期不匹配，即使您在方法內部使用任務。 我認為如果您將 Print 更改為返回 Task<string>，您會得到預期的結果：

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;
}

Answer 4

在處理延續時，我發現將我編寫 .ContinueWith 的位置視為執行立即繼續到它后面的語句的地方，而不是它“內部”的語句是很有用的。 在這種情況下，很明顯您將在 Send 中得到一個空字符串。 如果您對響應的唯一處理是將其寫入控制台，則在 Ito 的解決方案中您不需要任何等待 - 控制台打印輸出將無需等待，但在這種情況下 Send 和 Print 都應返回 void 。 在控制台應用程序中運行它，您將獲得頁面的打印輸出。

IMO、等待和 Task.Result 調用（哪個塊）有時是必要的，這取決於您想要的控制流，但更多時候它們表明您沒有真正正確地使用異步功能。

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);
            });
        }
    }
}

Answer 5

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;  
}  

Answer 6

回答標題的干凈示例

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

task.Wait();
Console.WriteLine(output);