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Async.StartImmediate Method (F#)

Den Delimarsky|Last Updated: 6/10/2016
1 Contributor

Runs an asynchronous computation, starting immediately on the current operating system thread.

Namespace/Module Path: Microsoft.FSharp.Control

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


// Signature:
static member StartImmediate : Async<unit> * CancellationToken option -> unit

// Usage:
Async.StartImmediate (computation)
Async.StartImmediate (computation, cancellationToken = cancellationToken)


computation Type: Async<unit>

The asynchronous computation to execute.

cancellationToken Type: CancellationToken

The optional cancellation token to associate with the computation. The default is used if this parameter is not provided.


If no cancellation token is provided then the default cancellation token is used.


The following code example shows how to use Async.StartImmediate to start an asynchronous computation on the current thread. Often, an asynchronous operation needs to update UI, which should always be done on the UI thread. When your asynchronous operation needs to begin by updating UI, Async.StartImmediate is a better choice than Async.Start, which starts the asynchronous operation on a thread pool thread.

    open System.Windows.Forms

    let bufferData = Array.zeroCreate<byte> 100000000

    let async1 (button : Button) =
         async {
           button.Text <- "Busy"
           button.Enabled <- false
           let context = System.Threading.SynchronizationContext.Current
           do! Async.SwitchToThreadPool()
           use outputFile = System.IO.File.Create("longoutput.dat")
           do! outputFile.AsyncWrite(bufferData)
           do! Async.SwitchToContext(context)
           button.Text <- "Start"
           button.Enabled <- true

    let form = new Form(Text = "Test Form")
    let button = new Button(Text = "Start")
    button.Click.Add(fun args -> Async.StartImmediate(async1 button))


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

Version Information

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

See Also

Control.Async Class (F#)

Microsoft.FSharp.Control Namespace (F#)

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