Gör så här: Förhindra att en underordnad uppgift ansluter till dess överordnade
Det här dokumentet visar hur du förhindrar att en underordnad uppgift kopplas till den överordnade aktiviteten. Det är användbart att förhindra att en underordnad uppgift kopplas till dess överordnade när du anropar en komponent som är skriven av en tredje part och som även använder uppgifter. En komponent från tredje part som använder TaskCreationOptions.AttachedToParent alternativet för att skapa ett objekt eller Task<TResult> ett Task objekt kan till exempel orsaka problem i koden om den är tidskrävande eller utlöser ett ohanterat undantag.
Exempel
I följande exempel jämförs effekterna av att använda standardalternativen med effekterna av att förhindra att en underordnad uppgift kopplas till den överordnade aktiviteten. Exemplet skapar ett Task objekt som anropar till ett bibliotek från tredje part som också använder ett Task objekt. Biblioteket från tredje part använder AttachedToParent alternativet för att skapa Task objektet. Programmet använder alternativet TaskCreationOptions.DenyChildAttach för att skapa den överordnade aktiviteten. Det här alternativet instruerar körningen att ta bort specifikationen AttachedToParent i underordnade uppgifter.
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.
'
Eftersom en överordnad aktivitet inte slutförs förrän alla underordnade aktiviteter har slutförts kan en långvarig underordnad aktivitet göra att det övergripande programmet fungerar dåligt. I det här exemplet, när programmet använder standardalternativen för att skapa den överordnade aktiviteten, måste den underordnade aktiviteten slutföras innan den överordnade aktiviteten är klar. När programmet använder TaskCreationOptions.DenyChildAttach alternativet är det underordnade inte kopplat till det överordnade objektet. Därför kan programmet utföra ytterligare arbete när den överordnade aktiviteten har slutförts och innan den måste vänta tills den underordnade aktiviteten har slutförts.
