HOW TO：處理工作擲回的例外狀況
下列範例顯示如何處理一個或多個工作所擲回的例外狀況。
範例
在這第一個範例中，會攔截到 System.AggregateException，然後檢查其 AggregateExceptionInnerExceptions()，以判斷是否有任何例外狀況可讓程式碼處理。
' How to: Handle Exceptions Thrown by Tasks
Imports System.Threading.Tasks
Module TaskExceptions
Function GetAllFiles(ByVal str As String) As String()
' Should throw an AccessDenied exception on Vista or later. If you see an "Exception was unhandled
' by user code" error, this is because "Just My Code" is enabled. Press F5 to continue execution or
' disable Just My Code.
Return System.IO.Directory.GetFiles(str, "*.txt", System.IO.SearchOption.AllDirectories)
End Function
Sub Main()
HandleExceptions()
' RethrowAllExceptions()
Console.WriteLine("Press any key.")
Console.ReadKey()
End Sub
Sub HandleExceptions()
' Assume this is a user-entered String.
Dim path = "C:\"
' Use this line to throw UnauthorizedAccessException, which we handle.
Dim task1 = Task(Of String()).Factory.StartNew(Function() GetAllFiles(path))
' Use this line to throw an exception that is not handled.
' Task task1 = Task.Factory.StartNew(Sub () throw new IndexOutOfRangeException() )
Try
task1.Wait()
Catch ae As AggregateException
ae.Handle(Function(x)
If TypeOf (x) Is UnauthorizedAccessException Then ' This we know how to handle
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 True
Else
Return False ' Let anything else stop the application.
End If
End Function)
End Try
Console.WriteLine("task1 has completed.")
End Sub
Function GetValidExtensions(ByVal path As String) As String()
If path = "C:\" Then
Throw New ArgumentException("The system root is not a valid path.")
End If
Dim result(10) As String
Return result
End Function
Sub RethrowAllExceptions()
' Assume this is a user-entered String.
Dim path = "C:\"
Dim myTasks(2) As Task(Of String())
myTasks(0) = Task(Of String()).Factory.StartNew(Function() GetAllFiles(path))
myTasks(1) = Task(Of String()).Factory.StartNew(Function() GetValidExtensions(path))
myTasks(2) = Task(Of String()).Factory.StartNew(Function()
Dim s(10) As String
Return s
End Function)
Try
Task.WaitAll(myTasks)
Catch ae As AggregateException
Throw ae.Flatten()
End Try
Console.WriteLine("task1 has completed.")
End Sub
End Module
static string[] GetAllFiles(string str)
{
// Should throw an AccessDenied exception on Vista.
return System.IO.Directory.GetFiles(str, "*.txt", System.IO.SearchOption.AllDirectories);
}
static void HandleExceptions()
{
// Assume this is a user-entered string.
string path = @"C:\";
// Use this line to throw UnauthorizedAccessException, which we handle.
Task<string[]> task1 = Task<string[]>.Factory.StartNew(() => GetAllFiles(path));
// Use this line to throw an exception that is not handled.
// Task task1 = Task.Factory.StartNew(() => { throw new IndexOutOfRangeException(); } );
try
{
task1.Wait();
}
catch (AggregateException ae)
{
ae.Handle((x) =>
{
if (x is UnauthorizedAccessException) // This we know how to handle.
{
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 true;
}
return false; // Let anything else stop the application.
});
}
Console.WriteLine("task1 has completed.");
}
在這個範例中，會攔截到 System.AggregateException，但不會嘗試處理其任何內部例外狀況。 相反地，會使用 Flatten 方法來擷取任何巢狀 AggregateException 執行個體中的內部例外狀況，然後重新擲回單一 AggregateException (其中直接包含所有未處理的內部例外狀況)。 將例外狀況簡維可讓用戶端程式碼更方便處理。
Imports System.Threading.Tasks
Module TaskExceptions2
Sub Main()
RethrowAllExceptions()
End Sub
Function GetAllFiles(ByVal str As String) As String()
' Should throw an AccessDenied exception on Vista or later. If you see an "Exception was unhandled
' by user code" error, this is because "Just My Code" is enabled. Press F5 to continue execution or
' disable Just My Code.
Return System.IO.Directory.GetFiles(str, "*.txt", System.IO.SearchOption.AllDirectories)
End Function
Function GetValidExtensions(ByVal path As String) As String()
If path = "C:\" Then
Throw New ArgumentException("The system root is not a valid path.")
End If
Dim result(10) As String
Return result
End Function
Sub RethrowAllExceptions()
' Assume this is a user-entered String.
Dim path = "C:\"
Dim myTasks(2) As Task(Of String())
myTasks(0) = Task(Of String()).Factory.StartNew(Function() GetAllFiles(path))
myTasks(1) = Task(Of String()).Factory.StartNew(Function() GetValidExtensions(path))
myTasks(2) = Task(Of String()).Factory.StartNew(Function()
Dim s(10) As String
Return s
End Function)
Try
Task.WaitAll(myTasks)
Catch ae As AggregateException
Throw ae.Flatten()
End Try
Console.WriteLine("task1 has completed.")
End Sub
End Module
static string[] GetValidExtensions(string path)
{
if (path == @"C:\")
throw new ArgumentException("The system root is not a valid path.");
return new string[10];
}
static void RethrowAllExceptions()
{
// Assume this is a user-entered string.
string path = @"C:\";
Task<string[]>[] tasks = new Task<string[]>[3];
tasks[0] = Task<string[]>.Factory.StartNew(() => GetAllFiles(path));
tasks[1] = Task<string[]>.Factory.StartNew(() => GetValidExtensions(path));
tasks[2] = Task<string[]>.Factory.StartNew(() => new string[10]);
//int index = Task.WaitAny(tasks2);
//double d = tasks2[index].Result;
try
{
Task.WaitAll(tasks);
}
catch (AggregateException ae)
{
throw ae.Flatten();
}
Console.WriteLine("task1 has completed.");
}
若要執行這個範例，請將程式碼貼至上一個範例中，然後在 Main 方法中呼叫 RethrowAllExceptions。