Async-await：线程会一直运行到我的等待吗？

Question

我一直认为如果我调用一个异步函数，线程就会开始执行这个异步函数，直到它看到一个等待。我认为它会向上调用堆栈查看调用者是否没有等待，而不是无所事事地等待。如果没有，则执行代码。

考虑以下（简化的）代码：

async Task<string> FetchCustomerNameAsync(int customerId)

{

    // check if customerId is positive:

    if (customerId <= 0) throw new ArgumentOutofRangeException(nameof(customerId);

    // fetch the Customer and return the name:

    Customer customer = await FetchCustomerAsync(customerId);

    return customer.Name;

}

现在，如果我的异步函数FetchCustomerNameAsync(+1)不等待就调用会发生什么：

var myTask = FetchCustmerNameAsync(+1);

DoSomethingElse();

string customerName = await myTask;

FetchCustomerNameAsync, 以 +1 的参数值调用

FetchCustomerNameAsync检测到它customerId是阳性的，所以也不例外

FetchCustomerNameAsync 电话 FetchCustomerAsync

里面的某个地方FetchCustomerAsync是一个等待。发生这种情况时，线程会向上调用调用堆栈，直到其中一个调用者没有等待。

FetchCustomerNameAsync 正在等待，所以调用堆栈

我的功能还没有等待，继续 DoSomethingElse()

我的功能满足等待。

我的想法是，在满足我函数中的 await 之前，已经完成了对参数值的检查。

因此，以下应在 await 之前导致异常：

// call with invalid parameter; do not await

var myTask = FetchCustmerNameAsync(-1);      // <-- note the minus 1!

Debug.Assert(false, "Exception expected");

我认为虽然我没有等待，但在Debug.Assert.

然而，在我的程序中，在 Debug.Assert为什么之前没有抛出异常？到底发生了什么？

Answer 1

异常仅在等待任务时传播您不能在不等待任务的情况下处理异常。异常仅在线程/任务内传播。因此，如果您不等待，异常只会停止任务。如果在您等待之前抛出异常，它将在您实际等待时传播。之前做所有的验证，然后做异步工作。所以，我建议你之前验证：ValidateId(id); // This will throw synchronously.Task<Customer> customer = FetchCustomerAsync(id).ConfigureAwait(false);DoSomethingElse();return await customer.Name;这是实现您想要的并行性的最佳方式。

Answer 2

您是对的，该线程执行异步函数直到它看到等待。事实上，你ArgumentOutofRangeException是由你调用的线程抛出的FetchCustmerNameAsync。即使它是同一个线程也不会得到异常的原因是因为当您await在函数内部使用时，AsyncStateMachine构建了 a 。它将所有代码转换为状态机，但重要的部分是它如何处理异常。看一看：这段代码：public void M() {&nbsp; &nbsp; var t = DoWork(1);}public async Task DoWork(int amount){&nbsp; &nbsp; if(amount == 1)&nbsp; &nbsp; &nbsp; &nbsp; throw new ArgumentException();&nbsp; &nbsp; await Task.Delay(1);}转换为（我跳过了不重要的部分）：private void MoveNext(){&nbsp; &nbsp; int num = <>1__state;&nbsp; &nbsp; try&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; TaskAwaiter awaiter;&nbsp; &nbsp; &nbsp; &nbsp; if (num != 0)&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (amount == 1)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; throw new ArgumentException();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; awaiter = Task.Delay(1).GetAwaiter();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (!awaiter.IsCompleted)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // Unimportant&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; else&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // Unimportant&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; catch (Exception exception)&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; <>1__state = -2;&nbsp; &nbsp; &nbsp; &nbsp; <>t__builder.SetException(exception); // Add exception to the task.&nbsp; &nbsp; &nbsp; &nbsp; return;&nbsp; &nbsp; }&nbsp; &nbsp; <>1__state = -2;&nbsp; &nbsp; <>t__builder.SetResult();}如果你跟着<>t__builder.SetException(exception);( AsyncMethodBuilder.SetException)，你会发现它最终会调用task.TrySetException(exception);which 将异常添加到任务的 中exceptionHolder，可以通过Task.Exception属性检索。

Answer 3

一个简化的 MCVE ：&nbsp; &nbsp; static async Task Main(string[] args)&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp;&nbsp; &nbsp; &nbsp; &nbsp; try&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; // enable 1 of these calls&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; var task = DoSomethingAsync();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //&nbsp; var task = DoSomethingTask();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Console.WriteLine("Still Ok");&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; await task;&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; catch (Exception ex)&nbsp; &nbsp; &nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Console.WriteLine(ex.Message);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; private static async Task DoSomethingAsync()&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; throw new NotImplementedException();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;&nbsp;&nbsp; &nbsp; }&nbsp; &nbsp; private static Task DoSomethingTask()&nbsp; &nbsp; {&nbsp; &nbsp; &nbsp; &nbsp; throw new NotImplementedException();&nbsp; &nbsp; &nbsp; &nbsp; return Task.CompletedTask;&nbsp; &nbsp; }当您调用 DoSomethingAsync 时，您将看到“Still Ok”消息。当您调用 DoSomethingTask 时，您将获得您期望的行为：WriteLine 之前的立即异常。