异常处理（任务并行库）
由在任务内部运行的用户代码引发的未处理异常会传播回调用线程，但本主题稍后部分介绍的某些情况除外。 如果使用静态或实例 Task.Wait 方法之一，异常会传播，异常处理方法为将调用封闭到 try/catch 语句中。 如果任务是所附加子任务的父级，或在等待多个任务，那么可能会引发多个异常。
为了将所有异常传播回调用线程，任务基础结构会将这些异常包装在 AggregateException 实例中。 AggregateException 异常具有 InnerExceptions 属性，可枚举该属性来检查引发的所有原始异常，并单独处理（或不处理）每个异常。 也可以使用 AggregateException.Handle 方法处理原始异常。
即使只引发了一个异常，仍会将该异常包装在 AggregateException 中，如以下示例所示。
public static partial class Program
{
public static void HandleThree()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
{
// Handle the custom exception.
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
// Rethrow any other exception.
else
{
throw ex;
}
}
}
}
}
// The example displays the following output:
// This exception is expected!
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))
Try
task1.Wait()
Catch ae As AggregateException
For Each ex In ae.InnerExceptions
' Handle the custom exception.
If TypeOf ex Is CustomException Then
Console.WriteLine(ex.Message)
' Rethrow any other exception.
Else
Throw ex
End If
Next
End Try
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays the following output:
' This exception is expected!
可以通过只捕获 AggregateException 而不观察任何内部异常来避免未处理的异常。 但是，我们建议你不要这样做，因为这样相当于在非并行情况下捕获基 Exception 类型。 捕获异常而不采取具体措施从中恢复可能会使程序进入不确定状态。
如果不想调用 Task.Wait 方法来等待任务完成，也可以通过任务的 Exception 属性检索 AggregateException 异常，如下面的示例所示。 有关详细信息，请参阅本主题中的通过使用 Task.Exception 属性观察异常部分。
public static partial class Program
{
public static void HandleFour()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
while (!task.IsCompleted) { }
if (task.Status == TaskStatus.Faulted)
{
foreach (var ex in task.Exception?.InnerExceptions ?? new(Array.Empty<Exception>()))
{
// Handle the custom exception.
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
// Rethrow any other exception.
else
{
throw ex;
}
}
}
}
}
// The example displays the following output:
// This exception is expected!
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))
While Not task1.IsCompleted
End While
If task1.Status = TaskStatus.Faulted Then
For Each ex In task1.Exception.InnerExceptions
' Handle the custom exception.
If TypeOf ex Is CustomException Then
Console.WriteLine(ex.Message)
' Rethrow any other exception.
Else
Throw ex
End If
Next
End If
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays the following output:
' This exception is expected!
注意
上述示例代码包含一个 while 循环，用于轮询任务的 Task.IsCompleted 属性以确定任务何时完成。 不应在生产代码中执行此操作，因为其效率非常低下。
如果不等待传播异常的任务，或要访问其 Exception 属性，则会在对该任务进行垃圾回收时根据 .NET 异常策略提升异常。
如果允许异常向上冒泡回到联接线程，则一个任务也许可以在引发异常后继续处理一些项。
注意
某些情况下，当启用“仅我的代码”后，Visual Studio 会在引发异常的行中断运行并显示一条错误消息，该消息显示“用户代码未处理异常”。该错误是良性错误。 可以按 F5 继续并查看在这些示例中演示的异常处理行为。 若要阻止 Visual Studio 在出现第一个错误时中断运行，只需在“工具”->“选项”->“调试”->“常规” 下取消选中“启用‘仅我的代码’” 复选框即可。
附加子任务和嵌套 AggregateExceptions
如果某个任务具有引发异常的附加子任务，则会在将该异常传播到父任务之前将其包装在 AggregateException 中，父任务将该异常包装在自己的 AggregateException 中，然后再将其传播回调用线程。 在这种情况下，在 Task.Wait、WaitAny、或 WaitAll 方法处捕获的 AggregateException 异常的 InnerExceptions 属性包含一个或多个 AggregateException 实例，而不包含导致错误的原始异常。 为了避免必须循环访问嵌套 AggregateException 异常，可以使用 Flatten 方法删除所有嵌套 AggregateException 异常，以便 AggregateException.InnerExceptions 属性包含原始异常。 在下面的示例中，嵌套 AggregateException 实例已经平展，并且仅在一个循环中处理。
public static partial class Program
{
public static void FlattenTwo()
{
var task = Task.Factory.StartNew(() =>
{
var child = Task.Factory.StartNew(() =>
{
var grandChild = Task.Factory.StartNew(() =>
{
// This exception is nested inside three AggregateExceptions.
throw new CustomException("Attached child2 faulted.");
}, TaskCreationOptions.AttachedToParent);
// This exception is nested inside two AggregateExceptions.
throw new CustomException("Attached child1 faulted.");
}, TaskCreationOptions.AttachedToParent);
});
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var ex in ae.Flatten().InnerExceptions)
{
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
else
{
throw;
}
}
}
}
}
// The example displays the following output:
// Attached child1 faulted.
// Attached child2 faulted.
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Factory.StartNew(Sub()
Dim child1 = Task.Factory.StartNew(Sub()
Dim child2 = Task.Factory.StartNew(Sub()
Throw New CustomException("Attached child2 faulted.")
End Sub,
TaskCreationOptions.AttachedToParent)
Throw New CustomException("Attached child1 faulted.")
End Sub,
TaskCreationOptions.AttachedToParent)
End Sub)
Try
task1.Wait()
Catch ae As AggregateException
For Each ex In ae.Flatten().InnerExceptions
If TypeOf ex Is CustomException Then
Console.WriteLine(ex.Message)
Else
Throw
End If
Next
End Try
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays the following output:
' Attached child1 faulted.
' Attached child2 faulted.
还可以使用 AggregateException.Flatten 方法，通过多个任务在一个 AggregateException 实例中抛出的多个 AggregateException 实例重新抛出内部异常，如下面的示例所示。
public static partial class Program
{
public static void TaskExceptionTwo()
{
try
{
ExecuteTasks();
}
catch (AggregateException ae)
{
foreach (var e in ae.InnerExceptions)
{
Console.WriteLine(
"{0}:\n {1}", e.GetType().Name, e.Message);
}
}
}
static void ExecuteTasks()
{
// Assume this is a user-entered String.
string path = @"C:\";
List<Task> tasks = new();
tasks.Add(Task.Run(() =>
{
// This should throw an UnauthorizedAccessException.
return Directory.GetFiles(
path, "*.txt",
SearchOption.AllDirectories);
}));
tasks.Add(Task.Run(() =>
{
if (path == @"C:\")
{
throw new ArgumentException(
"The system root is not a valid path.");
}
return new string[] { ".txt", ".dll", ".exe", ".bin", ".dat" };
}));
tasks.Add(Task.Run(() =>
{
throw new NotImplementedException(
"This operation has not been implemented.");
}));
try
{
Task.WaitAll(tasks.ToArray());
}
catch (AggregateException ae)
{
throw ae.Flatten();
}
}
}
// The example displays the following output:
// UnauthorizedAccessException:
// Access to the path 'C:\Documents and Settings' is denied.
// ArgumentException:
// The system root is not a valid path.
// NotImplementedException:
// This operation has not been implemented.
Imports System.Collections.Generic
Imports System.IO
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Try
ExecuteTasks()
Catch ae As AggregateException
For Each e In ae.InnerExceptions
Console.WriteLine("{0}:{2} {1}", e.GetType().Name, e.Message,
vbCrLf)
Next
End Try
End Sub
Sub ExecuteTasks()
' Assume this is a user-entered String.
Dim path = "C:\"
Dim tasks As New List(Of Task)
tasks.Add(Task.Run(Function()
' This should throw an UnauthorizedAccessException.
Return Directory.GetFiles(path, "*.txt",
SearchOption.AllDirectories)
End Function))
tasks.Add(Task.Run(Function()
If path = "C:\" Then
Throw New ArgumentException("The system root is not a valid path.")
End If
Return {".txt", ".dll", ".exe", ".bin", ".dat"}
End Function))
tasks.Add(Task.Run(Sub()
Throw New NotImplementedException("This operation has not been implemented.")
End Sub))
Try
Task.WaitAll(tasks.ToArray)
Catch ae As AggregateException
Throw ae.Flatten()
End Try
End Sub
End Module
' The example displays the following output:
' UnauthorizedAccessException:
' Access to the path 'C:\Documents and Settings' is denied.
' ArgumentException:
' The system root is not a valid path.
' NotImplementedException:
' This operation has not been implemented.
分离子任务中的异常
默认情况下，子任务在创建时处于分离状态。 必须在直接父任务中处理或重新引发从分离任务引发的异常；将不会采用与附加子任务传播回异常相同的方式将这些异常传播回调用线程。 最顶层的父级可以手动重新引发分离子级中的异常，以使其包装在 AggregateException 中并传播回调用线程。
public static partial class Program
{
public static void DetachedTwo()
{
var task = Task.Run(() =>
{
var nestedTask = Task.Run(
() => throw new CustomException("Detached child task faulted."));
// Here the exception will be escalated back to the calling thread.
// We could use try/catch here to prevent that.
nestedTask.Wait();
});
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var e in ae.Flatten().InnerExceptions)
{
if (e is CustomException)
{
Console.WriteLine(e.Message);
}
}
}
}
}
// The example displays the following output:
// Detached child task faulted.
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Run(Sub()
Dim nestedTask1 = Task.Run(Sub()
Throw New CustomException("Detached child task faulted.")
End Sub)
' Here the exception will be escalated back to joining thread.
' We could use try/catch here to prevent that.
nestedTask1.Wait()
End Sub)
Try
task1.Wait()
Catch ae As AggregateException
For Each ex In ae.Flatten().InnerExceptions
If TypeOf ex Is CustomException Then
' Recover from the exception. Here we just
' print the message for demonstration purposes.
Console.WriteLine(ex.Message)
End If
Next
End Try
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays the following output:
' Detached child task faulted.
即使使用延续观察子任务中的异常，该异常仍然必须由父任务观察。
指示协作取消的异常
在任务中的用户代码响应取消请求时，正确的过程是引发传入在其上传达请求的取消标记中的 OperationCanceledException 。 在尝试传播异常之前，任务实例会将异常中的标记与创建异常时传递给异常的标记进行比较。 如果标记相同，则任务会传播包装在 TaskCanceledException 中的 AggregateException，并且将可以在检查内部异常时看到它。 但是，如果调用线程未在等待任务，则将不会传播此特定异常。 有关详细信息，请参阅任务取消。
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
var task = Task.Factory.StartNew(() =>
{
CancellationToken ct = token;
while (someCondition)
{
// Do some work...
Thread.SpinWait(50_000);
ct.ThrowIfCancellationRequested();
}
},
token);
// No waiting required.
tokenSource.Dispose();
Dim someCondition As Boolean = True
Dim tokenSource = New CancellationTokenSource()
Dim token = tokenSource.Token
Dim task1 = Task.Factory.StartNew(Sub()
Dim ct As CancellationToken = token
While someCondition = True
' Do some work...
Thread.SpinWait(500000)
ct.ThrowIfCancellationRequested()
End While
End Sub,
token)
使用 Handle 方法筛选内部异常
可以使用 AggregateException.Handle 方法，筛选掉可视为“已处理”的异常，而无需进一步使用任何逻辑。 在提供给 AggregateException.Handle(Func<Exception,Boolean>) 方法的用户委托中，可以检查异常类型及其 Message 属性，或可便于确定异常是否为良性的其他任何信息。 在 AggregateException.Handle 方法返回结果后，便会立即在新实例 AggregateException 中重新抛出委托针对其返回 false 的任何异常。
下面的示例在功能上相当于本主题中的第一个示例（检查 AggregateException.InnerExceptions 集合中的所有异常）。 相反，此异常处理程序对每个异常调用 AggregateException.Handle 方法对象，并仅重新抛出不是 CustomException 实例的异常。
public static partial class Program
{
public static void HandleMethodThree()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
try
{
task.Wait();
}
catch (AggregateException ae)
{
// Call the Handle method to handle the custom exception,
// otherwise rethrow the exception.
ae.Handle(ex =>
{
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
return ex is CustomException;
});
}
}
}
// The example displays the following output:
// This exception is expected!
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))
Try
task1.Wait()
Catch ae As AggregateException
' Call the Handle method to handle the custom exception,
' otherwise rethrow the exception.
ae.Handle(Function(e)
If TypeOf e Is CustomException Then
Console.WriteLine(e.Message)
End If
Return TypeOf e Is CustomException
End Function)
End Try
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays the following output:
' This exception is expected!
下面是更完整的示例，在枚举文件时，使用 AggregateException.Handle 方法提供 UnauthorizedAccessException 异常的特殊处理。
public static partial class Program
{
public static void TaskException()
{
// This should throw an UnauthorizedAccessException.
try
{
if (GetAllFiles(@"C:\") is { Length: > 0 } files)
{
foreach (var file in files)
{
Console.WriteLine(file);
}
}
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
{
Console.WriteLine(
"{0}: {1}", ex.GetType().Name, ex.Message);
}
}
Console.WriteLine();
// This should throw an ArgumentException.
try
{
foreach (var s in GetAllFiles(""))
{
Console.WriteLine(s);
}
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
Console.WriteLine(
"{0}: {1}", ex.GetType().Name, ex.Message);
}
}
static string[] GetAllFiles(string path)
{
var task1 =
Task.Run(() => Directory.GetFiles(
path, "*.txt",
SearchOption.AllDirectories));
try
{
return task1.Result;
}
catch (AggregateException ae)
{
ae.Handle(x =>
{
// Handle an UnauthorizedAccessException
if (x is UnauthorizedAccessException)
{
Console.WriteLine(
"You do not have permission to access all folders in this path.");
Console.WriteLine(
"See your network administrator or try another path.");
}
return x is UnauthorizedAccessException;
});
return Array.Empty<string>();
}
}
}
// The example displays the following output:
// You do not have permission to access all folders in this path.
// See your network administrator or try another path.
//
// ArgumentException: The path is not of a legal form.
Imports System.IO
Imports System.Threading.Tasks
Module Example
Public Sub Main()
' This should throw an UnauthorizedAccessException.
Try
Dim files = GetAllFiles("C:\")
If files IsNot Nothing Then
For Each file In files
Console.WriteLine(file)
Next
End If
Catch ae As AggregateException
For Each ex In ae.InnerExceptions
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
Next
End Try
Console.WriteLine()
' This should throw an ArgumentException.
Try
For Each s In GetAllFiles("")
Console.WriteLine(s)
Next
Catch ae As AggregateException
For Each ex In ae.InnerExceptions
Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
Next
End Try
Console.WriteLine()
End Sub
Function GetAllFiles(ByVal path As String) As String()
Dim task1 = Task.Run(Function()
Return Directory.GetFiles(path, "*.txt",
SearchOption.AllDirectories)
End Function)
Try
Return task1.Result
Catch ae As AggregateException
ae.Handle(Function(x)
' Handle an UnauthorizedAccessException
If TypeOf x Is UnauthorizedAccessException Then
Console.WriteLine("You do not have permission to access all folders in this path.")
Console.WriteLine("See your network administrator or try another path.")
End If
Return TypeOf x Is UnauthorizedAccessException
End Function)
End Try
Return Array.Empty(Of String)()
End Function
End Module
' The example displays the following output:
' You do not have permission to access all folders in this path.
' See your network administrator or try another path.
'
' ArgumentException: The path is not of a legal form.
通过使用 Task.Exception 属性观察异常
如果任务完成时的状态为 TaskStatus.Faulted，可以检查它的 Exception 属性，以发现是哪个异常导致错误发生。 观察 Exception 属性的一个好方法是使用仅在前面的任务出错时才运行的延续，如以下示例所示。
public static partial class Program
{
public static void ExceptionPropagationTwo()
{
_ = Task.Run(
() => throw new CustomException("task1 faulted."))
.ContinueWith(_ =>
{
if (_.Exception?.InnerException is { } inner)
{
Console.WriteLine("{0}: {1}",
inner.GetType().Name,
inner.Message);
}
},
TaskContinuationOptions.OnlyOnFaulted);
Thread.Sleep(500);
}
}
// The example displays output like the following:
// CustomException: task1 faulted.
Imports System.Threading
Imports System.Threading.Tasks
Module Example
Public Sub Main()
Dim task1 = Task.Factory.StartNew(Sub()
Throw New CustomException("task1 faulted.")
End Sub).
ContinueWith(Sub(t)
Console.WriteLine("{0}: {1}",
t.Exception.InnerException.GetType().Name,
t.Exception.InnerException.Message)
End Sub, TaskContinuationOptions.OnlyOnFaulted)
Thread.Sleep(500)
End Sub
End Module
Class CustomException : Inherits Exception
Public Sub New(s As String)
MyBase.New(s)
End Sub
End Class
' The example displays output like the following:
' CustomException: task1 faulted.
在有意义的应用程序中，延续委托可能会记录有关异常的详细信息，并可能生成新任务以从异常中恢复。 如果任务出错，以下表达式将引发异常：
await tasktask.Wait()task.Resulttask.GetAwaiter().GetResult()
使用 try-catch 语句来处理和观察引发的异常。 或者，通过访问 Task.Exception 属性来观察异常。
UnobservedTaskException 事件
在某些情况下（例如承载不受信任的插件时），良性异常可能比较普遍，因此很难以手动方式观察到所有异常。 在这些情况下，可以处理 TaskScheduler.UnobservedTaskException 事件。 传递到处理程序的 System.Threading.Tasks.UnobservedTaskExceptionEventArgs 实例可用于阻止未观察到的异常传播回联接线程。
请参阅
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