Procedura: Evitare l'associazione di un'attività figlio alla relativa attività padre
In questo documento viene illustrato come impedire a un'attività figlio di essere associata all'attività padre. L'impedimento dell'associazione di un'attività figlio al relativo padre è utile quando si chiama un componente scritto da terze parti e in cui si utilizzano anche attività. Ad esempio, un componente di terze parti in cui si utilizza l'opzione TaskCreationOptions.AttachedToParent per creare un oggetto Task o Task<TResult> può causare problemi nel codice se è in esecuzione prolungata o genera un'eccezione non gestita.
Esempio
Nell'esempio seguente vengono confrontati gli effetti dell'utilizzo delle opzioni predefinite con gli effetti della mancata associazione di un'attività figlio al padre. Nell'esempio viene creato un oggetto Task tramite cui viene chiamata una libreria di terze parti in cui si utilizza anche un oggetto Task. Nella libreria di terze parti viene utilizzata l'opzione AttachedToParent per creare l'oggetto Task. Nell'applicazione viene utilizzata l'opzione TaskCreationOptions.DenyChildAttach per creare l'attività padre. Questa opzione indica al runtime di rimuovere la specifica AttachedToParent nelle attività figlio.
using System;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
// Defines functionality that is provided by a third-party.
// In a real-world scenario, this would likely be provided
// in a separate code file or assembly.
namespace Contoso
{
public class Widget
{
public Task Run()
{
// Create a long-running task that is attached to the
// parent in the task hierarchy.
return Task.Factory.StartNew(() =>
{
// Simulate a lengthy operation.
Thread.Sleep(5000);
}, TaskCreationOptions.AttachedToParent);
}
}
}
// Demonstrates how to prevent a child task from attaching to the parent.
class DenyChildAttach
{
static void RunWidget(Contoso.Widget widget,
TaskCreationOptions parentTaskOptions)
{
// Record the time required to run the parent
// and child tasks.
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
Console.WriteLine("Starting widget as a background task...");
// Run the widget task in the background.
Task<Task> runWidget = Task.Factory.StartNew(() =>
{
Task widgetTask = widget.Run();
// Perform other work while the task runs...
Thread.Sleep(1000);
return widgetTask;
}, parentTaskOptions);
// Wait for the parent task to finish.
Console.WriteLine("Waiting for parent task to finish...");
runWidget.Wait();
Console.WriteLine("Parent task has finished. Elapsed time is {0} ms.",
stopwatch.ElapsedMilliseconds);
// Perform more work...
Console.WriteLine("Performing more work on the main thread...");
Thread.Sleep(2000);
Console.WriteLine("Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds);
// Wait for the child task to finish.
Console.WriteLine("Waiting for child task to finish...");
runWidget.Result.Wait();
Console.WriteLine("Child task has finished. Elapsed time is {0} ms.",
stopwatch.ElapsedMilliseconds);
}
static void Main(string[] args)
{
Contoso.Widget w = new Contoso.Widget();
// Perform the same operation two times. The first time, the operation
// is performed by using the default task creation options. The second
// time, the operation is performed by using the DenyChildAttach option
// in the parent task.
Console.WriteLine("Demonstrating parent/child tasks with default options...");
RunWidget(w, TaskCreationOptions.None);
Console.WriteLine();
Console.WriteLine("Demonstrating parent/child tasks with the DenyChildAttach option...");
RunWidget(w, TaskCreationOptions.DenyChildAttach);
}
}
/* Sample output:
Demonstrating parent/child tasks with default options...
Starting widget as a background task...
Waiting for parent task to finish...
Parent task has finished. Elapsed time is 5014 ms.
Performing more work on the main thread...
Elapsed time is 7019 ms.
Waiting for child task to finish...
Child task has finished. Elapsed time is 7019 ms.
Demonstrating parent/child tasks with the DenyChildAttach option...
Starting widget as a background task...
Waiting for parent task to finish...
Parent task has finished. Elapsed time is 1007 ms.
Performing more work on the main thread...
Elapsed time is 3015 ms.
Waiting for child task to finish...
Child task has finished. Elapsed time is 5015 ms.
*/
Imports System.Diagnostics
Imports System.Threading
Imports System.Threading.Tasks
' Defines functionality that is provided by a third-party.
' In a real-world scenario, this would likely be provided
' in a separate code file or assembly.
Namespace Contoso
Public Class Widget
Public Function Run() As Task
' Create a long-running task that is attached to the
' parent in the task hierarchy.
Return Task.Factory.StartNew(Sub() Thread.Sleep(5000), TaskCreationOptions.AttachedToParent)
' Simulate a lengthy operation.
End Function
End Class
End Namespace
' Demonstrates how to prevent a child task from attaching to the parent.
Friend Class DenyChildAttach
Private Shared Sub RunWidget(ByVal widget As Contoso.Widget, ByVal parentTaskOptions As TaskCreationOptions)
' Record the time required to run the parent
' and child tasks.
Dim stopwatch As New Stopwatch()
stopwatch.Start()
Console.WriteLine("Starting widget as a background task...")
' Run the widget task in the background.
Dim runWidget As Task(Of Task) = Task.Factory.StartNew(Function()
' Perform other work while the task runs...
Dim widgetTask As Task = widget.Run()
Thread.Sleep(1000)
Return widgetTask
End Function, parentTaskOptions)
' Wait for the parent task to finish.
Console.WriteLine("Waiting for parent task to finish...")
runWidget.Wait()
Console.WriteLine("Parent task has finished. Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)
' Perform more work...
Console.WriteLine("Performing more work on the main thread...")
Thread.Sleep(2000)
Console.WriteLine("Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)
' Wait for the child task to finish.
Console.WriteLine("Waiting for child task to finish...")
runWidget.Result.Wait()
Console.WriteLine("Child task has finished. Elapsed time is {0} ms.", stopwatch.ElapsedMilliseconds)
End Sub
Shared Sub Main(ByVal args() As String)
Dim w As New Contoso.Widget()
' Perform the same operation two times. The first time, the operation
' is performed by using the default task creation options. The second
' time, the operation is performed by using the DenyChildAttach option
' in the parent task.
Console.WriteLine("Demonstrating parent/child tasks with default options...")
RunWidget(w, TaskCreationOptions.None)
Console.WriteLine()
Console.WriteLine("Demonstrating parent/child tasks with the DenyChildAttach option...")
RunWidget(w, TaskCreationOptions.DenyChildAttach)
End Sub
End Class
' Sample output:
'Demonstrating parent/child tasks with default options...
'Starting widget as a background task...
'Waiting for parent task to finish...
'Parent task has finished. Elapsed time is 5014 ms.
'Performing more work on the main thread...
'Elapsed time is 7019 ms.
'Waiting for child task to finish...
'Child task has finished. Elapsed time is 7019 ms.
'
'Demonstrating parent/child tasks with the DenyChildAttach option...
'Starting widget as a background task...
'Waiting for parent task to finish...
'Parent task has finished. Elapsed time is 1007 ms.
'Performing more work on the main thread...
'Elapsed time is 3015 ms.
'Waiting for child task to finish...
'Child task has finished. Elapsed time is 5015 ms.
'
Poiché un'attività padre non viene completata finché non lo sono anche tutte le attività figlio, un'attività figlio di lunga durata può ridurre notevolmente le prestazioni dell'applicazione in generale. In questo esempio, quando nell'applicazione vengono utilizzate le opzioni predefinite per creare l'attività padre, l'attività figlio deve essere completata prima di quella del padre. Quando nell'applicazione viene utilizzata l'opzione TaskCreationOptions.DenyChildAttach, l'elemento figlio non è associato al padre. Pertanto, tramite l'applicazione è possibile eseguire lavoro aggiuntivo al termine dell'attività padre e prima dell'attesa del completamento dell'attività figlio.
