Task.Run Method

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
 static System::Threading::Tasks::Task ^ Run(Func<System::Threading::Tasks::Task ^> ^ function, System::Threading::CancellationToken cancellationToken);

public static System.Threading.Tasks.Task Run(Func<System.Threading.Tasks.Task> function, System.Threading.CancellationToken cancellationToken);

public static System.Threading.Tasks.Task Run(Func<System.Threading.Tasks.Task?> function, System.Threading.CancellationToken cancellationToken);

static member Run : Func<System.Threading.Tasks.Task> * System.Threading.CancellationToken -> System.Threading.Tasks.Task

Public Shared Function Run (function As Func(Of Task), cancellationToken As CancellationToken) As Task

Parameters

Returns

A task that represents a proxy for the task returned by function.

Exceptions

Remarks

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Task Cancellation
• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
 static System::Threading::Tasks::Task ^ Run(Action ^ action, System::Threading::CancellationToken cancellationToken);

public static System.Threading.Tasks.Task Run(Action action, System.Threading.CancellationToken cancellationToken);

static member Run : Action * System.Threading.CancellationToken -> System.Threading.Tasks.Task

Public Shared Function Run (action As Action, cancellationToken As CancellationToken) As Task

Parameters

Returns

A task that represents the work queued to execute in the thread pool.

Exceptions

Examples

The following example calls the Run(Action, CancellationToken) method to create a task that iterates the files in the C:\Windows\System32 directory. The lambda expression calls the Parallel.ForEach method to add information about each file to a List<T> object. Each detached nested task invoked by the Parallel.ForEach 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.

using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
 public static async Task Main()
 {
 var tokenSource = new CancellationTokenSource();
 var token = tokenSource.Token;
 var files = new List<Tuple<string, string, long, DateTime>>();

 var t = Task.Run( () => { string dir = "C:\\Windows\\System32\\";
 object obj = new Object();
 if (Directory.Exists(dir)) {
 Parallel.ForEach(Directory.GetFiles(dir),
 f => {
 if (token.IsCancellationRequested)
 token.ThrowIfCancellationRequested();
 var fi = new FileInfo(f);
 lock(obj) {
 files.Add(Tuple.Create(fi.Name, fi.DirectoryName, fi.Length, fi.LastWriteTimeUtc)); 
 }
 });
 }
 }
 , token);
 await Task.Yield();
 tokenSource.Cancel();
 try {
 await t;
 Console.WriteLine("Retrieved information for {0} files.", files.Count);
 }
 catch (AggregateException e) {
 Console.WriteLine("Exception messages:");
 foreach (var ie in e.InnerExceptions)
 Console.WriteLine(" {0}: {1}", ie.GetType().Name, ie.Message);

 Console.WriteLine("\nTask status: {0}", t.Status); 
 }
 finally {
 tokenSource.Dispose();
 }
 }
}
// The example displays the following output:
// Exception messages:
// TaskCanceledException: A task was canceled.
// TaskCanceledException: A task was canceled.
// ...
// 
// Task status: Canceled

open System
open System.IO
open System.Threading
open System.Threading.Tasks

let main =
 task {
 use tokenSource = new CancellationTokenSource()
 let token = tokenSource.Token
 let files = ResizeArray()

 let t =
 Task.Run(
 (fun () ->
 let dir = "C:\\Windows\\System32\\"
 let obj = obj ()

 if Directory.Exists dir then
 Parallel.ForEach(
 Directory.GetFiles dir,
 (fun f ->
 if token.IsCancellationRequested then
 token.ThrowIfCancellationRequested()

 let fi = FileInfo f
 lock obj (fun () -> files.Add(fi.Name, fi.DirectoryName, fi.Length, fi.LastWriteTimeUtc)))
 )
 |> ignore),
 token
 )

 do! Task.Yield()
 tokenSource.Cancel()

 try
 do! t
 printfn $"Retrieved information for {files.Count} files."

 with :? AggregateException as e ->
 printfn "Exception messages:"

 for ie in e.InnerExceptions do
 printfn $" {ie.GetType().Name}: {ie.Message}"

 printfn $"Task status: {t.Status}"
 }

main.Wait()


// The example displays the following output:
// Exception messages:
// TaskCanceledException: A task was canceled.
// TaskCanceledException: A task was canceled.
// ...
//
// Task status: Canceled

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 Task = Task.Run( 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)
 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

Remarks

If cancellation is requested before the task begins execution, the task does not execute. Instead it is set to the Canceled state and throws a TaskCanceledException exception.

The Run(Action, CancellationToken) method is a simpler alternative to the TaskFactory.StartNew(Action, CancellationToken) method. It creates a task with the following default values:

• Its CreationOptions property value is TaskCreationOptions.DenyChildAttach.

• It uses the default task scheduler.

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
 static System::Threading::Tasks::Task ^ Run(Func<System::Threading::Tasks::Task ^> ^ function);

public static System.Threading.Tasks.Task Run(Func<System.Threading.Tasks.Task> function);

public static System.Threading.Tasks.Task Run(Func<System.Threading.Tasks.Task?> function);

static member Run : Func<System.Threading.Tasks.Task> -> System.Threading.Tasks.Task

Public Shared Function Run (function As Func(Of Task)) As Task

Parameters

Returns

A task that represents a proxy for the task returned by function.

Exceptions

Remarks

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
 static System::Threading::Tasks::Task ^ Run(Action ^ action);

public static System.Threading.Tasks.Task Run(Action action);

static member Run : Action -> System.Threading.Tasks.Task

Public Shared Function Run (action As Action) As Task

Parameters

Returns

A task that represents the work queued to execute in the ThreadPool.

Exceptions

Examples

The following example defines a ShowThreadInfo method that displays the Thread.ManagedThreadId of the current thread. It is called directly from the application thread, and is called from the Action delegate passed to the Run(Action) method.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 ShowThreadInfo("Application");

 var t = Task.Run(() => ShowThreadInfo("Task") );
 t.Wait();
 }

 static void ShowThreadInfo(String s)
 {
 Console.WriteLine("{0} thread ID: {1}",
 s, Thread.CurrentThread.ManagedThreadId);
 }
}
// The example displays the following output:
// Application thread ID: 1
// Task thread ID: 3

open System.Threading
open System.Threading.Tasks

let showThreadInfo s =
 printfn $"%s{s} thread ID: {Thread.CurrentThread.ManagedThreadId}"

showThreadInfo "Application"

let t = Task.Run(fun () -> showThreadInfo "Task")
t.Wait()

// The example displays the following output:
// Application thread ID: 1
// Task thread ID: 3

Imports System.Threading
Imports System.Threading.Tasks

Module Example
 Public Sub Main()
 ShowThreadInfo("Application")

 Dim t As Task = Task.Run(Sub() ShowThreadInfo("Task") )
 t.Wait()
 End Sub
 
 Private Sub ShowThreadInfo(s As String)
 Console.WriteLine("{0} Thread ID: {1}",
 s, Thread.CurrentThread.ManagedThreadId)
 End Sub
End Module
' The example displays output like the following:
' Application thread ID: 1
' Task thread ID: 3

The following example is similar to the previous one, except that it uses a lambda expression to define the code that the task is to execute.

using System;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 Console.WriteLine("Application thread ID: {0}",
 Thread.CurrentThread.ManagedThreadId);
 var t = Task.Run(() => { Console.WriteLine("Task thread ID: {0}",
 Thread.CurrentThread.ManagedThreadId);
 } );
 t.Wait();
 }
}
// The example displays the following output:
// Application thread ID: 1
// Task thread ID: 3

open System.Threading
open System.Threading.Tasks

printfn $"Application thread ID: {Thread.CurrentThread.ManagedThreadId}"
let t = Task.Run(fun () -> printfn $"Task thread ID: {Thread.CurrentThread.ManagedThreadId}")
t.Wait()

// The example displays the following output:
// Application thread ID: 1
// Task thread ID: 3

Imports System.Threading
Imports System.Threading.Tasks

Module Example
 Public Sub Main()
 Console.WriteLine("Application thread ID: {0}",
 Thread.CurrentThread.ManagedThreadId)
 Dim t As Task = Task.Run(Sub()
 Console.WriteLine("Task thread ID: {0}",
 Thread.CurrentThread.ManagedThreadId)
 End Sub)
 t.Wait()
 End Sub
End Module
' The example displays output like the following:
' Application thread ID: 1
' Task thread ID: 3

The examples show that the asynchronous task executes on a different thread than the main application thread.

The call to the Wait method ensures that the task completes and displays its output before the application ends. Otherwise, it is possible that the Main method will complete before the task finishes.

The following example illustrates the Run(Action) method. It defines an array of directory names and starts a separate task to retrieve the file names in each directory. All tasks write the file names to a single ConcurrentBag<T> object. The example then calls the WaitAll(Task[]) method to ensure that all tasks have completed, and then displays a count of the total number of file names written to the ConcurrentBag<T> object.

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.IO;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 var list = new ConcurrentBag<string>();
 string[] dirNames = { ".", ".." };
 List<Task> tasks = new List<Task>();
 foreach (var dirName in dirNames) {
 Task t = Task.Run( () => { foreach(var path in Directory.GetFiles(dirName)) 
 list.Add(path); } );
 tasks.Add(t);
 }
 Task.WaitAll(tasks.ToArray());
 foreach (Task t in tasks)
 Console.WriteLine("Task {0} Status: {1}", t.Id, t.Status);
 
 Console.WriteLine("Number of files read: {0}", list.Count);
 }
}
// The example displays output like the following:
// Task 1 Status: RanToCompletion
// Task 2 Status: RanToCompletion
// Number of files read: 23

open System.Collections.Concurrent
open System.IO
open System.Threading.Tasks

let list = ConcurrentBag<string>()
let dirNames = [ "."; ".." ]
let tasks = ResizeArray()

for dirName in dirNames do
 let t =
 Task.Run(fun () ->
 for path in Directory.GetFiles dirName do
 list.Add path)

 tasks.Add t

tasks.ToArray() |> Task.WaitAll

for t in tasks do
 printfn $"Task {t.Id} Status: {t.Status}"

printfn $"Number of files read: {list.Count}"

// The example displays output like the following:
// Task 1 Status: RanToCompletion
// Task 2 Status: RanToCompletion
// Number of files read: 23

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

Module Example
 Public Sub Main()
 Dim list As New ConcurrentBag(Of String)()
 Dim dirNames() As String = { ".", ".." }
 Dim tasks As New List(Of Task)()
 For Each dirName In dirNames 
 Dim t As Task = Task.Run( Sub()
 For Each path In Directory.GetFiles(dirName) 
 list.Add(path)
 Next
 End Sub )
 tasks.Add(t)
 Next
 Task.WaitAll(tasks.ToArray())
 For Each t In tasks
 Console.WriteLine("Task {0} Status: {1}", t.Id, t.Status)
 Next 
 Console.WriteLine("Number of files read: {0}", list.Count)
 End Sub
End Module
' The example displays output like the following:
' Task 1 Status: RanToCompletion
' Task 2 Status: RanToCompletion
' Number of files read: 23

Remarks

The Run method allows you to create and execute a task in a single method call and is a simpler alternative to the StartNew method. It creates a task with the following default values:

• Its cancellation token is CancellationToken.None.

• Its CreationOptions property value is TaskCreationOptions.DenyChildAttach.

• It uses the default task scheduler.

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
generic <typename TResult>
 static System::Threading::Tasks::Task<TResult> ^ Run(Func<System::Threading::Tasks::Task<TResult> ^> ^ function);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<System.Threading.Tasks.Task<TResult>> function);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<System.Threading.Tasks.Task<TResult>?> function);

static member Run : Func<System.Threading.Tasks.Task<'Result>> -> System.Threading.Tasks.Task<'Result>

Public Shared Function Run(Of TResult) (function As Func(Of Task(Of TResult))) As Task(Of TResult)

Type Parameters

Parameters

Returns

A Task(TResult) that represents a proxy for the Task(TResult) returned by function.

Exceptions

Remarks

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
generic <typename TResult>
 static System::Threading::Tasks::Task<TResult> ^ Run(Func<TResult> ^ function);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<TResult> function);

static member Run : Func<'Result> -> System.Threading.Tasks.Task<'Result>

Public Shared Function Run(Of TResult) (function As Func(Of TResult)) As Task(Of TResult)

Type Parameters

Parameters

Returns

A task object that represents the work queued to execute in the thread pool.

Exceptions

Examples

The following example counts the approximate number of words in text files that represent published books. Each task is responsible for opening a file, reading its entire contents asynchronously, and calculating the word count by using a regular expression. The WaitAll(Task[]) method is called to ensure that all tasks have completed before displaying the word count of each book to the console.

using System;
using System.IO;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 string pattern = @"\p{P}*\s+";
 string[] titles = { "Sister Carrie", "The Financier" };
 Task<int>[] tasks = new Task<int>[titles.Length];

 for (int ctr = 0; ctr < titles.Length; ctr++) {
 string s = titles[ctr];
 tasks[ctr] = Task.Run( () => {
 // Number of words.
 int nWords = 0;
 // Create filename from title.
 string fn = s + ".txt";
 if (File.Exists(fn)) {
 StreamReader sr = new StreamReader(fn);
 string input = sr.ReadToEndAsync().Result;
 nWords = Regex.Matches(input, pattern).Count;
 }
 return nWords;
 } );
 }
 Task.WaitAll(tasks);

 Console.WriteLine("Word Counts:\n");
 for (int ctr = 0; ctr < titles.Length; ctr++)
 Console.WriteLine("{0}: {1,10:N0} words", titles[ctr], tasks[ctr].Result);
 }
}
// The example displays the following output:
// Sister Carrie: 159,374 words
// The Financier: 196,362 words

open System
open System.IO
open System.Text.RegularExpressions
open System.Threading.Tasks

let pattern = @"\p{P}*\s+"
let titles = [| "Sister Carrie"; "The Financier" |]

let tasks =
 Array.map (fun title ->
 Task.Run(fun () ->
 // Create filename from title.
 let fn = title + ".txt"

 if File.Exists fn then
 use sr = new StreamReader(fn)
 let input = sr.ReadToEndAsync().Result
 Regex.Matches(input, pattern).Count
 else
 0)) titles

tasks |> Seq.cast |> Array.ofSeq |> Task.WaitAll

printfn "Word Counts:\n"

for i = 0 to tasks.Length - 1 do
 printfn $"%s{titles.[i]}: %10d{tasks.[i].Result} words"

// The example displays the following output:
// Sister Carrie: 159,374 words
// The Financier: 196,362 words

Imports System.IO
Imports System.Text.RegularExpressions
Imports System.Threading.Tasks

Module Example
 Public Sub Main()
 Dim pattern As String = "\p{P}*\s+"
 Dim titles() As String = { "Sister Carrie",
 "The Financier" }
 Dim tasks(titles.Length - 1) As Task(Of Integer)

 For ctr As Integer = 0 To titles.Length - 1
 Dim s As String = titles(ctr)
 tasks(ctr) = Task.Run( Function()
 ' Number of words.
 Dim nWords As Integer = 0
 ' Create filename from title.
 Dim fn As String = s + ".txt"
 If File.Exists(fn) Then
 Dim sr As New StreamReader(fn)
 Dim input As String = sr.ReadToEndAsync().Result
 nWords = Regex.Matches(input, pattern).Count
 End If
 Return nWords
 End Function)
 Next
 Task.WaitAll(tasks)

 Console.WriteLine("Word Counts:")
 Console.WriteLine()
 For ctr As Integer = 0 To titles.Length - 1
 Console.WriteLine("{0}: {1,10:N0} words", titles(ctr), tasks(ctr).Result)
 Next
 End Sub
End Module
' The example displays the following output:
' Sister Carrie: 159,374 words
' The Financier: 196,362 words

The regular expression \p{P}*\s+ matches zero, one, or more punctuation characters followed by one or more white-space characters. It assumes that the total number of matches equals the approximate word count.

Remarks

The Run method is a simpler alternative to the TaskFactory.StartNew(Action) method. It creates a task with the following default values:

• Its cancellation token is CancellationToken.None.

• Its CreationOptions property value is TaskCreationOptions.DenyChildAttach.

• It uses the default task scheduler.

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
generic <typename TResult>
 static System::Threading::Tasks::Task<TResult> ^ Run(Func<System::Threading::Tasks::Task<TResult> ^> ^ function, System::Threading::CancellationToken cancellationToken);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<System.Threading.Tasks.Task<TResult>> function, System.Threading.CancellationToken cancellationToken);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<System.Threading.Tasks.Task<TResult>?> function, System.Threading.CancellationToken cancellationToken);

static member Run : Func<System.Threading.Tasks.Task<'Result>> * System.Threading.CancellationToken -> System.Threading.Tasks.Task<'Result>

Public Shared Function Run(Of TResult) (function As Func(Of Task(Of TResult)), cancellationToken As CancellationToken) As Task(Of TResult)

Type Parameters

Parameters

Returns

A Task(TResult) that represents a proxy for the Task(TResult) returned by function.

Exceptions

Remarks

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Task Cancellation
• Exception Handling (Task Parallel Library)

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

Source:

Task.cs

public:
generic <typename TResult>
 static System::Threading::Tasks::Task<TResult> ^ Run(Func<TResult> ^ function, System::Threading::CancellationToken cancellationToken);

public static System.Threading.Tasks.Task<TResult> Run<TResult>(Func<TResult> function, System.Threading.CancellationToken cancellationToken);

static member Run : Func<'Result> * System.Threading.CancellationToken -> System.Threading.Tasks.Task<'Result>

Public Shared Function Run(Of TResult) (function As Func(Of TResult), cancellationToken As CancellationToken) As Task(Of TResult)

Type Parameters

Parameters

Returns

A Task(TResult) that represents the work queued to execute in the thread pool.

Exceptions

Examples

The following example creates 20 tasks that will loop until a counter is incremented to a value of 2 million. When the first 10 tasks reach 2 million, the cancellation token is cancelled, and any tasks whose counters have not reached 2 million are cancelled. The example shows possible output.

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 var tasks = new List<Task<int>>();
 var source = new CancellationTokenSource();
 var token = source.Token;
 int completedIterations = 0;

 for (int n = 0; n <= 19; n++)
 tasks.Add(Task.Run( () => { int iterations = 0;
 for (int ctr = 1; ctr <= 2000000; ctr++) {
 token.ThrowIfCancellationRequested();
 iterations++;
 }
 Interlocked.Increment(ref completedIterations);
 if (completedIterations >= 10)
 source.Cancel();
 return iterations; }, token));

 Console.WriteLine("Waiting for the first 10 tasks to complete...\n");
 try {
 Task.WaitAll(tasks.ToArray());
 }
 catch (AggregateException) {
 Console.WriteLine("Status of tasks:\n");
 Console.WriteLine("{0,10} {1,20} {2,14:N0}", "Task Id",
 "Status", "Iterations");
 foreach (var t in tasks)
 Console.WriteLine("{0,10} {1,20} {2,14}",
 t.Id, t.Status,
 t.Status != TaskStatus.Canceled ? t.Result.ToString("N0") : "n/a");
 }
 }
}
// The example displays output like the following:
// Waiting for the first 10 tasks to complete...
// Status of tasks:
//
// Task Id Status Iterations
// 1 RanToCompletion 2,000,000
// 2 RanToCompletion 2,000,000
// 3 RanToCompletion 2,000,000
// 4 RanToCompletion 2,000,000
// 5 RanToCompletion 2,000,000
// 6 RanToCompletion 2,000,000
// 7 RanToCompletion 2,000,000
// 8 RanToCompletion 2,000,000
// 9 RanToCompletion 2,000,000
// 10 Canceled n/a
// 11 Canceled n/a
// 12 Canceled n/a
// 13 Canceled n/a
// 14 Canceled n/a
// 15 Canceled n/a
// 16 RanToCompletion 2,000,000
// 17 Canceled n/a
// 18 Canceled n/a
// 19 Canceled n/a
// 20 Canceled n/a

open System
open System.Collections.Generic
open System.Threading
open System.Threading.Tasks

let source = new CancellationTokenSource()
let token = source.Token
let mutable completedIterations = 0

let tasks =
 [| for _ = 0 to 19 do
 Task.Run(
 (fun () ->
 let mutable iterations = 0

 for _ = 1 to 2000000 do
 token.ThrowIfCancellationRequested()
 iterations <- iterations + 1

 Interlocked.Increment &completedIterations |> ignore

 if completedIterations >= 10 then
 source.Cancel()

 iterations),
 token
 )

 |]

printfn "Waiting for the first 10 tasks to complete...\n"

try
 tasks |> Seq.cast |> Array.ofSeq |> Task.WaitAll
with :? AggregateException ->
 printfn "Status of tasks:\n"
 printfn "%10s %20s %14s" "Task Id" "Status" "Iterations"

 for t in tasks do
 if t.Status <> TaskStatus.Canceled then
 t.Result.ToString "N0"
 else
 "n/a"
 |> printfn "%10i %20O %14s" t.Id t.Status


// The example displays output like the following:
// Waiting for the first 10 tasks to complete...
// Status of tasks:
//
// Task Id Status Iterations
// 1 RanToCompletion 2,000,000
// 2 RanToCompletion 2,000,000
// 3 RanToCompletion 2,000,000
// 4 RanToCompletion 2,000,000
// 5 RanToCompletion 2,000,000
// 6 RanToCompletion 2,000,000
// 7 RanToCompletion 2,000,000
// 8 RanToCompletion 2,000,000
// 9 RanToCompletion 2,000,000
// 10 Canceled n/a
// 11 Canceled n/a
// 12 Canceled n/a
// 13 Canceled n/a
// 14 Canceled n/a
// 15 Canceled n/a
// 16 RanToCompletion 2,000,000
// 17 Canceled n/a
// 18 Canceled n/a
// 19 Canceled n/a
// 20 Canceled n/a

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

Module Example

 Public Sub Main()
 Dim tasks As New List(Of Task(Of Integer))()
 Dim source As New CancellationTokenSource
 Dim token As CancellationToken = source.Token
 Dim completedIterations As Integer = 0
 
 For n As Integer = 0 To 19
 tasks.Add(Task.Run( Function()
 Dim iterations As Integer= 0
 For ctr As Long = 1 To 2000000
 token.ThrowIfCancellationRequested()
 iterations += 1
 Next
 Interlocked.Increment(completedIterations)
 If completedIterations >= 10 Then source.Cancel()
 Return iterations
 End Function, token))
 Next

 Console.WriteLine("Waiting for the first 10 tasks to complete... ")
 Try
 Task.WaitAll(tasks.ToArray())
 Catch e As AggregateException
 Console.WriteLine("Status of tasks:")
 Console.WriteLine()
 Console.WriteLine("{0,10} {1,20} {2,14}", "Task Id",
 "Status", "Iterations")
 For Each t In tasks
 Console.WriteLine("{0,10} {1,20} {2,14}",
 t.Id, t.Status,
 If(t.Status <> TaskStatus.Canceled,
 t.Result.ToString("N0"), "n/a"))
 Next
 End Try
 End Sub
End Module
' The example displays output like the following:
' Waiting for the first 10 tasks to complete...
' Status of tasks:
'
' Task Id Status Iterations
' 1 RanToCompletion 2,000,000
' 2 RanToCompletion 2,000,000
' 3 RanToCompletion 2,000,000
' 4 RanToCompletion 2,000,000
' 5 RanToCompletion 2,000,000
' 6 RanToCompletion 2,000,000
' 7 RanToCompletion 2,000,000
' 8 RanToCompletion 2,000,000
' 9 RanToCompletion 2,000,000
' 10 Canceled n/a
' 11 Canceled n/a
' 12 Canceled n/a
' 13 Canceled n/a
' 14 Canceled n/a
' 15 Canceled n/a
' 16 RanToCompletion 2,000,000
' 17 Canceled n/a
' 18 Canceled n/a
' 19 Canceled n/a
' 20 Canceled n/a

Instead of using the InnerExceptions property to examine exceptions, the example iterates all tasks to determine which have completed successfully and which have been cancelled. For those that have completed, it displays the value returned by the task.

Because cancellation is cooperative, each task can decide how to respond to cancellation. The following example is like the first, except that, once the token is cancelled, tasks return the number of iterations they've completed rather than throw an exception.

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

public class Example
{
 public static void Main()
 {
 var tasks = new List<Task<int>>();
 var source = new CancellationTokenSource();
 var token = source.Token;
 int completedIterations = 0;

 for (int n = 0; n <= 19; n++)
 tasks.Add(Task.Run( () => { int iterations = 0;
 for (int ctr = 1; ctr <= 2000000; ctr++) {
 if (token.IsCancellationRequested)
 return iterations;
 iterations++;
 }
 Interlocked.Increment(ref completedIterations);
 if (completedIterations >= 10)
 source.Cancel();
 return iterations; }, token));

 Console.WriteLine("Waiting for the first 10 tasks to complete...\n");
 try {
 Task.WaitAll(tasks.ToArray());
 }
 catch (AggregateException) {
 Console.WriteLine("Status of tasks:\n");
 Console.WriteLine("{0,10} {1,20} {2,14:N0}", "Task Id",
 "Status", "Iterations");
 foreach (var t in tasks)
 Console.WriteLine("{0,10} {1,20} {2,14}",
 t.Id, t.Status,
 t.Status != TaskStatus.Canceled ? t.Result.ToString("N0") : "n/a");
 }
 }
}
// The example displays output like the following:
// Status of tasks:
//
// Task Id Status Iterations
// 1 RanToCompletion 2,000,000
// 2 RanToCompletion 2,000,000
// 3 RanToCompletion 2,000,000
// 4 RanToCompletion 2,000,000
// 5 RanToCompletion 2,000,000
// 6 RanToCompletion 2,000,000
// 7 RanToCompletion 2,000,000
// 8 RanToCompletion 2,000,000
// 9 RanToCompletion 2,000,000
// 10 RanToCompletion 1,658,326
// 11 RanToCompletion 1,988,506
// 12 RanToCompletion 2,000,000
// 13 RanToCompletion 1,942,246
// 14 RanToCompletion 950,108
// 15 RanToCompletion 1,837,832
// 16 RanToCompletion 1,687,182
// 17 RanToCompletion 194,548
// 18 Canceled Not Started
// 19 Canceled Not Started
// 20 Canceled Not Started

open System
open System.Collections.Generic
open System.Threading
open System.Threading.Tasks

let source = new CancellationTokenSource()
let token = source.Token
let mutable completedIterations = 0

let tasks =
 [| for _ = 0 to 19 do
 Task.Run(
 (fun () ->
 let mutable iterations = 0

 for _ = 1 to 2000000 do
 token.ThrowIfCancellationRequested()
 iterations <- iterations + 1

 Interlocked.Increment &completedIterations |> ignore

 if completedIterations >= 10 then
 source.Cancel()

 iterations),
 token
 ) |]

printfn "Waiting for the first 10 tasks to complete...\n"

try
 tasks |> Seq.cast |> Array.ofSeq |> Task.WaitAll

with :? AggregateException ->
 printfn "Status of tasks:\n"
 printfn "%10s %20s %14s" "Task Id" "Status" "Iterations"

 for t in tasks do
 if t.Status <> TaskStatus.Canceled then
 t.Result.ToString "N0"
 else
 "n/a"
 |> printfn "%10i %20O %14s" t.Id t.Status

// The example displays output like the following:
// Status of tasks:
//
// Task Id Status Iterations
// 1 RanToCompletion 2,000,000
// 2 RanToCompletion 2,000,000
// 3 RanToCompletion 2,000,000
// 4 RanToCompletion 2,000,000
// 5 RanToCompletion 2,000,000
// 6 RanToCompletion 2,000,000
// 7 RanToCompletion 2,000,000
// 8 RanToCompletion 2,000,000
// 9 RanToCompletion 2,000,000
// 10 RanToCompletion 1,658,326
// 11 RanToCompletion 1,988,506
// 12 RanToCompletion 2,000,000
// 13 RanToCompletion 1,942,246
// 14 RanToCompletion 950,108
// 15 RanToCompletion 1,837,832
// 16 RanToCompletion 1,687,182
// 17 RanToCompletion 194,548
// 18 Canceled Not Started
// 19 Canceled Not Started
// 20 Canceled Not Started

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

Module Example

 Public Sub Main()
 Dim tasks As New List(Of Task(Of Integer))()
 Dim source As New CancellationTokenSource
 Dim token As CancellationToken = source.Token
 Dim completedIterations As Integer = 0
 
 For n As Integer = 0 To 19
 tasks.Add(Task.Run( Function()
 Dim iterations As Integer= 0
 For ctr As Long = 1 To 2000000
 If token.IsCancellationRequested Then
 Return iterations
 End If
 iterations += 1
 Next
 Interlocked.Increment(completedIterations)
 If completedIterations >= 10 Then source.Cancel()
 Return iterations
 End Function, token))
 Next

 Try
 Task.WaitAll(tasks.ToArray())
 Catch e As AggregateException
 Console.WriteLine("Status of tasks:")
 Console.WriteLine()
 Console.WriteLine("{0,10} {1,20} {2,14:N0}", "Task Id",
 "Status", "Iterations")
 For Each t In tasks
 Console.WriteLine("{0,10} {1,20} {2,14}",
 t.Id, t.Status,
 If(t.Status <> TaskStatus.Canceled,
 t.Result.ToString("N0"), "Not Started"))
 Next
 End Try
 End Sub
End Module
' The example displays output like the following:
' Status of tasks:
'
' Task Id Status Iterations
' 1 RanToCompletion 2,000,000
' 2 RanToCompletion 2,000,000
' 3 RanToCompletion 2,000,000
' 4 RanToCompletion 2,000,000
' 5 RanToCompletion 2,000,000
' 6 RanToCompletion 2,000,000
' 7 RanToCompletion 2,000,000
' 8 RanToCompletion 2,000,000
' 9 RanToCompletion 2,000,000
' 10 RanToCompletion 1,658,326
' 11 RanToCompletion 1,988,506
' 12 RanToCompletion 2,000,000
' 13 RanToCompletion 1,942,246
' 14 RanToCompletion 950,108
' 15 RanToCompletion 1,837,832
' 16 RanToCompletion 1,687,182
' 17 RanToCompletion 194,548
' 18 Canceled Not Started
' 19 Canceled Not Started
' 20 Canceled Not Started

The example still must handle the AggregateException exception, since any tasks that have not started when cancellation is requested still throw an exception.

Remarks

If cancellation is requested before the task begins execution, the task does not execute. Instead it is set to the Canceled state and throws a TaskCanceledException exception.

The Run method is a simpler alternative to the StartNew method. It creates a task with the following default values:

• Its CreationOptions property value is TaskCreationOptions.DenyChildAttach.

• It uses the default task scheduler.

For information on handling exceptions thrown by task operations, see Exception Handling.

See also

• Task Cancellation
• Exception Handling (Task Parallel Library)