Async.TryCancelled<'T> Method (F#)

Creates an asynchronous computation that executes the specified computation function. If this computation is cancelled before it completes then the computation generated by running the compensation function is executed.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
static member TryCancelled : Async<'T> * (OperationCanceledException -> unit) -> Async<'T>

// Usage:
Async.TryCancelled (computation, compensation)

Parameters

• computation
  Type: Async<'T>

  The input asynchronous computation.

• compensation
  Type: OperationCanceledException -> unit

  The function to be run if the computation is cancelled.

Return Value

An asynchronous computation that runs the compensation function if the input computation is cancelled.

Example

The following code example illustrates how to use Async.TryCancelled to run a cancellable computation.

open System
open System.Windows.Forms

let form = new Form(Text = "Test Form", Width = 400, Height = 400)
let panel1 = new Panel(Dock = DockStyle.Fill)
panel1.DockPadding.All <- 10
let spacing = 5
let startAsyncButton = new Button(Text = "Start", Enabled = true)
let controlHeight = startAsyncButton.Height
let button2 = new Button(Text = "Start Invalid", Top = controlHeight + spacing)
let cancelAsyncButton = new Button(Text = "Cancel",
                                   Top = 2 * (controlHeight + spacing),
                                   Enabled = false)
let updown1 = new System.Windows.Forms.NumericUpDown(Top = 3 * (controlHeight + spacing), 
                                                     Value = 20m, Minimum = 0m,
                                                     Maximum = 1000000m)
let label1 = new Label (Text = "", Top = 4 * (controlHeight + spacing),
                        Width = 300, Height = 2 * controlHeight)
let progressBar = new ProgressBar(Top = 6 * (controlHeight + spacing),
                                  Width = 300)
panel1.Controls.AddRange [| startAsyncButton; button2; cancelAsyncButton;
                            updown1; label1; progressBar; |]
form.Controls.Add(panel1)

// Recursive isprime function.
let isprime number =
    let rec check count =
        count > number/2 || (number % count <> 0 && check (count + 1))
    check 2

let isprimeBigInt number =
    let rec check count =
        count > number/2I || (number % count <> 0I && check (count + 1I))
    check 2I

let computeNthPrime (number) =
     if (number < 1) then
         invalidOp <| sprintf "Invalid input for nth prime: %s." (number.ToString())
     let mutable count = 0
     let mutable num = 1I
     let isDone = false
     while (count < number) do
         num <- num + 1I
         if (num < bigint System.Int32.MaxValue) then
             while (not (isprime (int num))) do
                 num <- num + 1I
         else
             while (not (isprimeBigInt num)) do
                 num <- num + 1I
         count <- count + 1
     num

let async1 context value =
    let asyncTryWith =
        async {
                    try
                        let nthPrime = ref 0I
                        for count in 1 .. value - 1 do
                            // The cancellation check is implicit and
                            // cooperative at for!, do!, and so on.
                            nthPrime := computeNthPrime(count)
                            // Report progress as a percentage of the total task.
                            let percentComplete = (int)((float)count /
                                                        (float)value * 100.0)
                            do! Async.SwitchToContext(context)
                            progressBar.Value <- percentComplete
                            do! Async.SwitchToThreadPool()
                        // Handle the case in which the operation succeeds.
                        do! Async.SwitchToContext(context)
                        label1.Text <- sprintf "%s" ((!nthPrime).ToString())
                    with 
                        | e -> 
                            // Handle the case in which an exception is thrown.
                            do! Async.SwitchToContext(context)
                            MessageBox.Show(e.Message) |> ignore
                }
    async {
        try
            do! Async.TryCancelled(asyncTryWith,
                                   (fun oce -> 
                                      // Handle the case in which the user cancels the operation.
                                      context.Post((fun _ ->
                                          label1.Text <- "Canceled"), null)))
        finally 
            context.Post((fun _ ->
                updown1.Enabled <- true
                startAsyncButton.Enabled <- true
                cancelAsyncButton.Enabled <- false),
                null)
    }

startAsyncButton.Click.Add(fun args -> 
    cancelAsyncButton.Enabled <- true
    let context = System.Threading.SynchronizationContext.Current
    Async.Start(async1 context (int updown1.Value)))
button2.Click.Add(fun args ->
   let context = System.Threading.SynchronizationContext.Current
   Async.Start(async1 context (int (-updown1.Value))))
cancelAsyncButton.Click.Add(fun args -> Async.CancelDefaultToken())
Application.Run(form)

Platforms

Windows 7, Windows Vista SP2, Windows XP SP3, Windows XP x64 SP2, Windows Server 2008 R2, Windows Server 2008 SP2, Windows Server 2003 SP2

Version Information

F# Runtime

Supported in: 2.0, 4.0

Silverlight

Supported in: 3

See Also

Reference

Control.Async Class (F#)

Microsoft.FSharp.Control Namespace (F#)