Async.CancelDefaultToken Method (F#)

Raises the cancellation condition for the most recent set of asynchronous computations started without any specific cancellation token. Replaces the global CancellationTokenSource object with a new global token source for any asynchronous computations created after this point without any specific cancellation token.

Namespace/Module Path: Microsoft.FSharp.Control

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
static member CancelDefaultToken : unit -> unit

// Usage:
Async.CancelDefaultToken ()

The following example shows how to create a cancellable asynchronous operation in a Windows Forms application. It also shows how to use Async.CancelDefaultToken to cancel the operation.

open System.Windows.Forms

let bufferData = Array.zeroCreate<byte> 100000000

let async1 (label:System.Windows.Forms.Label) filename =
     Async.StartWithContinuations(
         async {
            label.Text <- "Operation started." 
            use outputFile = System.IO.File.Create(filename)
            do! outputFile.AsyncWrite(bufferData)
            },
         (fun _ -> label.Text <- "Operation completed."),
         (fun _ -> label.Text <- "Operation failed."),
         (fun _ -> label.Text <- "Operation canceled."))



let form = new Form(Text = "Test Form")
let button1 = new Button(Text = "Start")
let button2 = new Button(Text = "Start Invalid", Top = button1.Height + 10)
let button3 = new Button(Text = "Cancel", Top = 2 * button1.Height + 20)
let label1 = new Label(Text = "", Width = 200, Top = 3 * button1.Height + 30)
form.Controls.AddRange [| button1; button2; button3; label1 |]
button1.Click.Add(fun args -> async1 label1 "longoutput.dat")
// Try an invalid filename to test the error case.
button2.Click.Add(fun args -> async1 label1 "|invalid.dat")
button3.Click.Add(fun args -> Async.CancelDefaultToken())
Application.Run(form)

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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