StartNew Method (Action, CancellationToken)

TaskFactory.StartNew Method (Action, CancellationToken)

 

Creates and starts a Task.

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

public Task StartNew(
	Action action,
	CancellationToken cancellationToken
)

Parameters

action
Type: System.Action

The action delegate to execute asynchronously.

cancellationToken
Type: System.Threading.CancellationToken

The CancellationToken that will be assigned to the new task.

Return Value

Type: System.Threading.Tasks.Task

The started Task.

Exception Condition
ObjectDisposedException

The provided CancellationToken has already been disposed.

ArgumentNullException

The exception that is thrown when the action argument is null.

Calling StartNew is functionally equivalent to creating a Task using one of its constructors and then calling Start to schedule it for execution.

Starting with the .NET Framework 4.5, you can use the Task.Run(Action, CancellationToken) method as a quick way to call StartNew(Action, CancellationToken) with default parameters. Note, however, that there is a difference in behavior between the two methods regarding : Task.Run(Action, CancellationToken) by default does not allow child tasks started with the TaskCreationOptions.AttachedToParent option to attach to the current Task instance, whereas StartNew(Action, CancellationToken) does. For more information and code examples, see Task.Run vs Task.Factory.StartNew in the Parallel Programming with .NET blog.

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

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

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

      var t = Task.Factory.StartNew( () => { 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);
      tokenSource.Cancel();
      try {
         t.Wait(); 
         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.
//       
//       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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