Async.SwitchToContext Method (F#)

Creates an asynchronous computation that runs its continuation using the Post method on the synchronization context object.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
static member SwitchToContext : SynchronizationContext -> Async<unit>

// Usage:
Async.SwitchToContext (syncContext)


Return Value

An asynchronous computation that uses the syncContext context to execute.


If syncContext is null then the asynchronous computation is equivalent to Async.SwitchToThreadPool.


The following code example illustrates how to use Async.SwitchToContext to switch to the UI thread to update the UI. In this, case a progress bar that indicates the state of completion of a computation is updated.

open System.Windows.Forms

let form = new Form(Text = "Test Form", Width = 400, Height = 400)
let syncContext = System.Threading.SynchronizationContext.Current
let button1 = new Button(Text = "Start")
let label1 = new Label(Text = "", Height = 200, Width = 200,
                       Top = button1.Height + 10)
form.Controls.AddRange([| button1; label1 |] )

type invokeFunction = delegate of unit -> unit

let async1(syncContext, form : System.Windows.Forms.Form) =
    async {
        let label1 = form.Controls.[1]
        // Do something.
        do! Async.Sleep(1000)

        // Switch to the UI thread and update the UI.
        do! Async.SwitchToContext(syncContext)

        let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
        label1.Text <- label1.Text + sprintf "On the UI Thread [%d]\n" threadNumber

        // Switch back to the thread pool.
        do! Async.SwitchToThreadPool()
        // Do something.
        do! Async.Sleep(1000)

        let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId

        // When on a thread pool thread, use Control.Invoke to update UI.
        label1.Invoke(new invokeFunction(fun () -> 
            label1.Text <- label1.Text +
                       sprintf "Switched to thread pool [%d]\n" threadNumber)) |> ignore

let buttonClick(sender:obj, args) =
    let button = sender :?> Button
    Async.Start(async1(syncContext, button.Parent :?> Form))

    let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
    label1.Text <- sprintf "Started asynchronous workflow [%d]\n"  threadNumber

button1.Click.AddHandler(fun sender args -> buttonClick(sender, args))


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


Supported in: 3

See Also


Control.Async Class (F#)

Microsoft.FSharp.Control Namespace (F#)