我试图理解为什么下面的代码：

async void Handle_Clicked(object sender, System.EventArgs e)
{
    try
    {
        await CrashAsync("aaa");
    }
    catch (Exception exception)
    {
        Log($"observed exception");

        Log($"Exception: {exception.Message}");
    }
}

private async Task CrashAsync(string title)
{
    Log($"CrashAsync ({title}) - before");

    await Task.Delay(1000);

    throw new Exception($"CrashAsync ({title})");

    Log($"CrashAsync ({title}) - after");
}

产生预期的结果：

thread #1: CrashAsync (aaa) – before

thread #1: observed exception

thread #1: Exception: CrashAsync (aaa)

但如果我将其更改为此：

async void Handle_Clicked(object sender, System.EventArgs e)
{
    try
    {
        await CrashAsync("aaa").ContinueWith(async (t) =>
        {                   
            await CrashAsync("bbb");
        },TaskContinuationOptions.OnlyOnRanToCompletion);
    }
    catch (Exception exception)
    {
        Log($"observed exception");

        Log($"Exception: {exception.Message}");
    }
}

我得到以下输出：

thread #1: CrashAsync (aaa) – before

thread #1: observed exception

thread #1: Exception: A task was canceled.

thread #2: unobserved exception

thread #2: System.Exception: CrashAsync (aaa)
at AsyncTest.AsyncTestPage+c__async3.MoveNext () [0x000ad] in /Users/johndoe/Development/Xamarin/AsyncTest/AsyncTest/AsyncTestPage.xaml.cs:82

哪里：

TaskScheduler.UnobservedTaskException += (sender, e) =>
{
    Debug.WriteLine($"thread #{Environment.CurrentManagedThreadId}: unobserved exception");

    foreach (var exception in e.Exception.Flatten().InnerExceptions)
    {
        Debug.WriteLine($"thread #{Environment.CurrentManagedThreadId}: {exception}");
    }
};

连续条件不满足,因此ContinueWith任务被取消,但是为什么我有未观察到的异常？

解决方法:

您等待由ContinueWith返回的Task,因此您观察到与该Task相关的异常-该异常已被取消(TaskCanceledException).但是,您不会观察到CrashAsync引发的原始异常(即“ CrashAsync aaa”)异常,因此不会观察到行为.

以下是示例代码,可以帮助您进一步理解：

static async void Test() {
    var originalTask = CrashAsync("aaa");
    var onSuccess = originalTask.ContinueWith(async (t) =>
    {
        await CrashAsync("bbb");
    }, TaskContinuationOptions.OnlyOnRanToCompletion);
    var onFault = originalTask.ContinueWith(t => {                    
        Log("Observed original exception: " + t.Exception.InnerExceptions[0].Message);
    }, TaskContinuationOptions.OnlyOnFaulted);
}

简而言之-等待您的任务并捕获异常(如果有).您根本不需要使用ContinueWith,因为如果您使用await,则该方法的其余部分已经是延续.