Como: Manipular exceções lançadas por tarefas
Os exemplos a seguir mostram como manipular exceções lançadas a partir de uma ou mais tarefas.
Exemplo
Neste primeiro exemplo, o System.AggregateException é capturado e sua AggregateExceptionInnerExceptions() são examinados para verificar se existem quaisquer exceções que podem ser tratadas pelo nosso código de programa.
' 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.");
}
Neste exemplo, o System.AggregateException é detectada, mas nenhuma tentativa é feita para lidar com qualquer uma das suas exceções inner. Em vez disso, o Flatten método é usado para extrair as exceções internas de qualquer aninhado AggregateException re-throw um único e instâncias AggregateException diretamente que contém todos os o interno sem tratamento exceções. A exceção de nivelamento torna mais fácil para o código de cliente para manipular.
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.");
}
Para executar esse exemplo, cole o código do exemplo anterior e a chamada RethrowAllExceptions partir do Main método.