Gewusst wie: Behandeln von Ausnahmen, die von Aufgaben ausgelöst werden
In den folgenden Beispielen wird gezeigt, wie Sie von einer oder mehreren Aufgaben ausgelöste Ausnahmen behandeln.
Beispiel
In diesem ersten Beispiel wird die System.AggregateException erfasst, und anschließend werden die zugehörigen AggregateExceptionInnerExceptions() analysiert, um festzustellen, ob Ausnahmen mithilfe des Programmcodes behandelt werden können.
' 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.");
}
In diesem Beispiel wird System.AggregateException erfasst, es wird jedoch nicht versucht, eine der inneren Ausnahmen zu behandeln. Stattdessen werden die inneren Ausnahmen mithilfe der Flatten-Methode aus geschachtelten AggregateException-Instanzen extrahiert, und es wird eine einzelne AggregateException erneut ausgelöst, die alle inneren nicht behandelten Ausnahmen enthält. Durch das Abflachen der Ausnahme kann diese vom Clientcode leichter behandelt werden.
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.");
}
Fügen Sie den Code in das vorherige Beispiel ein, und rufen Sie RethrowAllExceptions in der Main-Methode auf, um dieses Beispiel auszuführen.