async.switchtocontext-method-[fsharp].md

title	Async.SwitchToContext Method (F#)
description	Async.SwitchToContext Method (F#)
keywords	visual f#, f#, functional programming
author	dend
manager	danielfe
ms.date	05/16/2016
ms.topic	language-reference
ms.prod	visual-studio-dev14
ms.technology	devlang-fsharp
ms.assetid	c9ac8eb9-456c-4719-b017-b63d5eddb80a

Async.SwitchToContext Method (F#)

Creates an asynchronous computation that runs its continuation using the System.Threading.SynchronizationContext.Post(SendOrPostCallback,Object) method on the synchronization context object.

Namespace/Module Path: Microsoft.FSharp.Control

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

Syntax

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

// Usage:
Async.SwitchToContext (syncContext)

Parameters

syncContext Type: System.Threading.SynchronizationContext

The synchronization context to accept the posted computation.

Return Value

Returns an asynchronous computation that uses the syncContext context to execute.

Remarks

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

Example

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()
let button1 = new Button(Text = "Start")
let label1 = new Label(Text = "", Height = 200, Width = 200,
                       Top = button1.Height + 10)
form.Controls.AddRange([| button1; label1 |] )

let async1(syncContext, form : System.Windows.Forms.Form) =
    async {
        let label1 = form.Controls.[1]
        // Do something. 
        do! Async.Sleep(10000)
        let threadName = System.Threading.Thread.CurrentThread.Name
        let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
        label1.Text <- label1.Text + sprintf "Something [%s] [%d]" threadName threadNumber

        // Switch to the UI thread and update the UI. 
        do! Async.SwitchToContext(syncContext)
        let threadName = System.Threading.Thread.CurrentThread.Name
        let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
        label1.Text <- label1.Text + sprintf "Here [%s] [%d]" threadName threadNumber

        // Switch back to the thread pool. 
        do! Async.SwitchToThreadPool()
        // Do something. 
        do! Async.Sleep(10000)
        let threadName = System.Threading.Thread.CurrentThread.Name
        let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
        label1.Text <- label1.Text +
                       sprintf "Switched to thread pool [%s] [%d]" threadName threadNumber
    }

let buttonClick(sender:obj, args) =
    let button = sender :?> Button
    Async.Start(async1(syncContext, button.Parent :?> Form))
    let threadName = System.Threading.Thread.CurrentThread.Name
    let threadNumber = System.Threading.Thread.CurrentThread.ManagedThreadId
    button.Parent.Text <- sprintf "Started asynchronous workflow [%s] [%d]" threadName threadNumber
    ()

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

Platforms

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#)