c# - 问题与 Task.WhenAll 并行调用 Azure 移动客户端

Question

我的 Xamarin Android 应用程序有问题，但我相信我已将问题缩小到 Azure 移动客户端或我对 .Net 中的任务缺乏了解。我试图在控制台应用程序上重现我的问题。

我的目标是并行进行多个 api 调用。在下面的示例中，我只是重复调用以检索数据库中确实存在的用户。当我等待每次调用(GetUsersB 方法)时，一切都运行良好，但是当我尝试等待 Task.WhenAll(GetUsersA 方法)时，我几乎总是会遇到异常。

class Program
{     
    static  void Main(string[] args)
    {
        var service = new MyService();
        try
        {
            //GetUsersA(service).Wait(); //Often throws the exception attached but ocasionally is successful
            GetUsersB(service).Wait();   //Never throws an exception
        }
        catch (AggregateException e)
        {
            foreach (var ex in e.InnerExceptions)
            {
                Console.WriteLine(e.InnerException.Message);
                Console.WriteLine(e.InnerException.StackTrace);
            }
        }
        Console.WriteLine("main done");
        Console.ReadLine();
    }

    public async static Task GetUsersA(MyService service)
    {
        await Task.WhenAll(service.GetUser("d48977fce3c6fc6b5a74c"),
           service.GetUser("d48977fce3c6fc6b5a74c"),
           service.GetUser("d48977fce3c6fc6b5a74c"),             
           service.GetUser("d48977fce3c6fc6b5a74c"),
           service.GetUser("d48977fce3c6fc6b5a74c"),
           service.GetUser("d48977fce3c6fc6b5a74c"));
        Console.WriteLine("tasks complete");
    }
    public async static Task GetUsersB(MyService service)
    {
        await service.GetUser("d48977fce3c6fc6b5a74c");
        await service.GetUser("d48977fce3c6fc6b5a74c");         
        await service.GetUser("d48977fce3c6fc6b5a74c");
        await service.GetUser("d48977fce3c6fc6b5a74c");
        await service.GetUser("d48977fce3c6fc6b5a74c");
        Console.WriteLine("tasks complete");

    }
}   

如果需要，这里是 MyService 类

public class MyService
{
    private MobileServiceClient azClient;
    public MyService()
    {           
        azClient = new MobileServiceClient("https://mysite.azurewebsites.net/");           
    }

    public async Task<User> GetUser(string id)
    {
        return await azClient.GetTable<User>().LookupAsync(id);
    }
}

这是内部异常的输出:

Collection was modified; enumeration operation may not execute.

at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
at Microsoft.WindowsAzure.MobileServices.MobileServiceContractResolver.CreateProperties(Type type, MemberSerialization memberSerialization)
at Newtonsoft.Json.Serialization.DefaultContractResolver.CreateObjectContract(Type objectType)
at Microsoft.WindowsAzure.MobileServices.MobileServiceContractResolver.CreateObjectContract(Type type)
at Newtonsoft.Json.Serialization.DefaultContractResolver.CreateContract(Type objectType)
at Newtonsoft.Json.Serialization.DefaultContractResolver.ResolveContract(Type type)
at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
at Newtonsoft.Json.Linq.JToken.ToObject(Type objectType, JsonSerializer jsonSerializer)
at Newtonsoft.Json.Linq.JToken.ToObject[T](JsonSerializer jsonSerializer)
at Microsoft.WindowsAzure.MobileServices.MobileServiceSerializer.<>c__DisplayClass35_0`1.<Deserialize>b__0()
at Microsoft.WindowsAzure.MobileServices.MobileServiceSerializer.TransformSerializationException[T](Action action, JToken token)
at Microsoft.WindowsAzure.MobileServices.MobileServiceSerializer.Deserialize[T](JToken json, JsonSerializer jsonSerializer)
at Microsoft.WindowsAzure.MobileServices.MobileServiceSerializer.Deserialize[T](JToken json)
at Microsoft.WindowsAzure.MobileServices.MobileServiceTable`1.<LookupAsync>d__14.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.WindowsAzure.MobileServices.MobileServiceTable`1.<LookupAsync>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at TaskTest.MyService.<GetUser>d__2.MoveNext() in C:\Users\jalley\Documents\Visual Studio 2015\Projects\TaskTest\TaskTest\MyService.cs:line 24
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at TaskTest.Program.<GetUsersA>d__1.MoveNext() in C:\Users\jalley\Documents\Visual Studio 2015\Projects\TaskTest\TaskTest\Program.cs:line 31

Answer 1

查看 source code (第 307 行)您可以很容易地看出这是 MobileServiceTable<T> 的内部设计问题。 ，这似乎不是线程安全的(我不熟悉 Xamarin)。由于您使用的是控制台应用程序，因此所有任务都被卸载到线程池，并且并行性可能(并且将会)发生在第 307 行。从这一点开始，您有多个线程迭代同一个字典，而其他线程修改它。如果你再看看MobileServiceContractResolver ，您会看到一些使用“id”属性作为内部使用的缓存键的序列化，它可能与代码中的“d48977fce3c6fc6b5a74c”键相同。

这不是一个完整的答案，因为您的代码 看起来还不错。我认为这里不会有“真正的答案”，因为即使您可能过早地使用 async/await，您的代码也是完全有效的。我只是觉得您可能想知道为什么会发生这种情况。

还

这个库可能用于单线程上下文之上，例如 ASP.NET 或 UI 应用程序，而不是控制台应用程序。