Task Constructor (Action, CancellationToken)

 

Initializes a new Task with the specified action and CancellationToken.

Namespace:   System.Threading.Tasks
Assembly:  mscorlib (in mscorlib.dll)

Public Sub New (
	action As Action,
	cancellationToken As CancellationToken
)

Parameters

action
Type: System.Action

The delegate that represents the code to execute in the task.

cancellationToken
Type: System.Threading.CancellationToken

The CancellationToken that the new task will observe.

Exception Condition
ObjectDisposedException

The provided CancellationToken has already been disposed.

ArgumentNullException

The action argument is null.

Rather than calling this constructor, the most common way to instantiate a Task object and launch a task is by calling the static Task.Run(Action, CancellationToken) and TaskFactory.StartNew(Action, CancellationToken) methods. The only advantage offered by this constructor is that it allows object instantiation to be separated from task invocation.

For more information, see Task Parallelism (Task Parallel Library) and Cancellation in Managed Threads.

The following example calls the Task(Action, CancellationToken) constructor to create a task that iterates the files in the C:\Windows\System32 directory. The lambda expression calls the Parallel.ForEach(Of TSource) method to add information about each file to a List(Of T) object. Each detached nested task invoked by the Parallel.ForEach(Of TSource) loop checks the state of the cancellation token and, if cancellation is requested, calls the CancellationToken.ThrowIfCancellationRequested method. The CancellationToken.ThrowIfCancellationRequested method throws an OperationCanceledException exception that is handled in a catch block when the calling thread calls the Task.Wait method. The Start method is then called to start the task.

Imports System.Collections.Generic
Imports System.IO
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim tokenSource As New CancellationTokenSource()
      Dim token As CancellationToken = tokenSource.Token
      Dim files As New List(Of Tuple(Of String, String, Long, Date))()
      Dim t As New Task(Sub()
                           Dim dir As String = "C:\Windows\System32\"
                           Dim obj As New Object()
                           If Directory.Exists(dir)Then
                              Parallel.ForEach(Directory.GetFiles(dir), 
                                 Sub(f)
                                    If token.IsCancellationRequested Then
                                       token.ThrowIfCancellationRequested()
                                    End If  
                                    Dim fi As New FileInfo(f)
                                    SyncLock(obj)
                                       files.Add(Tuple.Create(fi.Name, fi.DirectoryName, fi.Length, fi.LastWriteTimeUtc))          
                                    End SyncLock
                                 End Sub)
                           End If
                        End Sub, token)
      t.Start()
      tokenSource.Cancel()
      Try
         t.Wait() 
         Console.WriteLine("Retrieved information for {0} files.", files.Count)
      Catch e As AggregateException
         Console.WriteLine("Exception messages:")
         For Each ie As Exception In e.InnerExceptions
            Console.WriteLine("   {0}:{1}", ie.GetType().Name, ie.Message)
         Next
         Console.WriteLine()
         Console.WriteLine("Task status: {0}", t.Status)       
      Finally
         tokenSource.Dispose()
      End Try
   End Sub
End Module
' The example displays the following output:
'       Exception messages:
'          TaskCanceledException: A task was canceled.
'       
'       Task status: Canceled

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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