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MailboxProcessor.Start<'Msg> Method (F#)

Creates and starts an agent.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
static member Start : (MailboxProcessor<'Msg> -> Async<unit>) * ?CancellationToken -> MailboxProcessor<'Msg>

// Usage:
MailboxProcessor.Start (body)
MailboxProcessor.Start (body, cancellationToken = cancellationToken)

body

Type: MailboxProcessor<'Msg> -> Async<unit>

The function to produce an asynchronous computation that will be executed as the read loop for the MailboxProcessor when Start is called.

cancellationToken

Type: CancellationToken

An optional cancellation token for the body. The default is Async.DefaultCancellationToken.

The body function is used to generate the asynchronous computation executed by the agent.

The following code example shows how to start a mailbox processor agent. In this example, each line of input from the console is posted to a message queue. The program reads each message and replies by using a reply channel. When the special message "Stop" is received, the Stop reply is sent and the program exits.

open System

type Message = string * AsyncReplyChannel<string>

let formatString = "Message number {0} was received. Message contents: {1}" 

let printThreadId note =

    // Append the thread ID.
    printfn "%d : %s" System.Threading.Thread.CurrentThread.ManagedThreadId note


let agent = MailboxProcessor<Message>.Start(fun inbox ->
    let rec loop n =
        async {
                let! (message, replyChannel) = inbox.Receive();
                printThreadId "MailboxProcessor" 
                if (message = "Stop") then
                    replyChannel.Reply("Stopping.")
                else
                    replyChannel.Reply(String.Format(formatString, n, message))
                do! loop (n + 1)
        }
    loop 0)

printfn "Mailbox Processor Test"
printfn "Type some text and press Enter to submit a message."
printfn "Type 'Stop' to close the program." 

let rec loop() =
    printf "> " 
    let input = Console.ReadLine()
    printThreadId("Console loop")
    let reply = agent.PostAndReply(fun replyChannel -> input, replyChannel)
    if (reply <> "Stopping.") then
        printfn "Reply: %s" reply
        loop()
    else
        ()
loop()

printfn "Press Enter to continue."
Console.ReadLine() |> ignore

Following is an example session.

Mailbox Processor Test
Type some text and press Enter to submit a message.
Type 'Stop' to close the program.
> hello
1 : Console loop
4 : mailboxProcessor
Reply: Message number 0 was received. Message contents: hello
> testing
1 : Console loop
3 : mailboxProcessor
Reply: Message number 1 was received. Message contents: testing
> hello?
1 : Console loop
4 : mailboxProcessor
Reply: Message number 2 was received. Message contents: hello?
> testing 1 2 3
1 : Console loop
3 : mailboxProcessor
Reply: Message number 3 was received. Message contents: testing 1 2 3
> Stop
1 : Console loop
4 : mailboxProcessor
Press Enter to continue.

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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