Task.WhenAll ContinueWith 死锁

Question

我在 xamarin 中开发我的移动应用程序并发送 x 个请求以显示我的应用程序的主页。

我的代码如下：

    var taskcoll = _mainArticlesAppService.GetWelcomeStartColl().ContinueWith(async (res) =>
        {
            _applicationContext.Categories = res.Result.Categories;
        });

        var taskheros = _mainArticlesAppService.GetWelcomeHerosProfileDto().ContinueWith((res) =>
        {
            _applicationContext.HerosProfileDto = res.Result;
            RaisePropertyChanged(() => HerosProfileDto);
        });

        List<Task> tasksInit = new List<Task>();

        tasksInit.Add(taskcoll);
        tasksInit.Add(taskheros);

        await Task.WhenAll(tasksInit).ConfigureAwait(false);

当没有问题时，一切都很好。

但是，当请求无法连接到远程 api 服务器时，它会在我的代码中引发 ecxeption，然后死锁，我无法再做任何事情并且我的屏幕被冻结。

我发生死锁的代码是：

 protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        int count = 0;

       using (var cts = new CancellationTokenSource(new TimeSpan(0, 0, 30)))
        {
            HttpResponseMessage response = null;
            try
            {
                
                 response = await base.SendAsync(request, cts.Token);
            }
            catch(Exception ex)
            {
               
                throw ex; // --> Deadlock
            }
            if (response.StatusCode == HttpStatusCode.Unauthorized &&
                HasBearerAuthorizationHeader(request))
            {
                return await HandleUnauthorizedResponse(request, response, cancellationToken);
            }

            return response;
        }
    }

Answer 1

不要将async/await 与延续混合，这会使代码变得不可预测。

同样没有延续代码看起来更好：

var taskcoll = _mainArticlesAppService.GetWelcomeStartColl(); // start 1st request
var taskheros = _mainArticlesAppService.GetWelcomeHerosProfileDto(); // start 2nd request
// both requests are already running
_applicationContext.Categories = (await taskcoll).Categories; // await 1st request
_applicationContext.HerosProfileDto = await taskheros; // await 2nd one
RaisePropertyChanged(() => HerosProfileDto);

也试试这个

response = await base.SendAsync(request, cts.Token).ConfigureAwait(false);

对于throw ex，确保在请求之外使用try-catch 处理Exception。当您按原样重新抛出 Exception 并且不执行任何其他操作时，在 SendAsync 内部，您根本不需要 try-catch。

---

这是一个带有多个异步调用的抽象示例，这些调用以不同的延迟返回数据。没有返回类型，但输入了 Actions 来进行回调。这种方法对我来说最准确，可以通过完成每个请求尽快接收到不同类型的数据。

class Program
{
    static async Task Main(string[] args)
    {
        SomeController controller = new SomeController();
        await controller.LoadAllData();
        Console.ReadKey();
    }  
}

public class SomeController
{
    private FirstDataClass _firstProperty;
    private SecondDataClass _secondProperty;
    private ThirdDataClass _thirdProperty;

    public FirstDataClass FirstProperty
    {
        get => _firstProperty;
        set
        {
            _firstProperty = value;
            Console.WriteLine("First data received");
        }
    }

    public SecondDataClass SecondProperty
    {
        get => _secondProperty;
        set
        {
            _secondProperty = value;
            Console.WriteLine("Second data received");
        }
    }
       
    public ThirdDataClass ThirdProperty
    {
        get => _thirdProperty;
        set
        {
            _thirdProperty = value;
            Console.WriteLine("Third data received");
        }
    }

    public async Task LoadAllData()
    {
        List<Task> tasks = new List<Task>();
        tasks.Add(GetFirstData(data => FirstProperty = data));
        tasks.Add(GetSecondData(data => SecondProperty = data));
        tasks.Add(GetThirdData(data => ThirdProperty = data));
        Console.WriteLine("Tasks launched");
        await Task.WhenAll(tasks);
    }

    private async Task GetFirstData(Action<FirstDataClass> action)
    {
        await Task.Delay(1000);
        action(new FirstDataClass());
    }

    private async Task GetSecondData(Action<SecondDataClass> action)
    {
        await Task.Delay(300);
        action(new SecondDataClass());
    }

    private async Task GetThirdData(Action<ThirdDataClass> action)
    {
        await Task.Delay(500);
        action(new ThirdDataClass());
    }
}

public class FirstDataClass { }

public class SecondDataClass { }

public class ThirdDataClass { }

控制台输出

Tasks launched
Second data received
Third data received
First data received

如果您不想更改 API 调用方法，您可以将此方法用于该方法的包装器，例如：

private async Task GetSomeData(Action<SomeType> action)
{
    action(await SomeApiCall());
}

或一些通用包装器

private async Task GetDataWrapper<T>(Func<Task<T>> method, Action<T> action)
{
    action(await method);
}