MailboxProcessor.TryPostAndReply<'Msg,'Reply> Method (F#)

Updated: January 2011

Like MailboxProcessor.PostAndReply, but returns None if there is no reply within the timeout period.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
member this.TryPostAndReply : (AsyncReplyChannel<'Reply> -> 'Msg) * ?int -> 'Reply option

// Usage:
mailboxProcessor.TryPostAndReply (buildMessage)
mailboxProcessor.TryPostAndReply (buildMessage, timeout = timeout)


Type: AsyncReplyChannel<'Reply> -> 'Msg

The function to incorporate the AsyncReplyChannel into the message to be sent.


Type: int

An optional timeout parameter (in milliseconds) to wait for a reply message. Defaults to -1 which corresponds to Infinite().

The reply from the agent or None if the timeout expires.

The following example shows how to use TryPostAndReply. In this example, the agent has a delay that eventually results in a timeout.

open System

type Message = string * AsyncReplyChannel<string>

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

let agent = MailboxProcessor<Message>.Start(fun inbox ->
    let rec loop n =
        async {
                let! (message, replyChannel) = inbox.Receive();
                // The delay gets longer with each message, and eventually will trigger a timeout.
                do! Async.Sleep(200 * n );
                if (message = "Stop") then
                    replyChannel.Reply(String.Format(formatString, n, message))
                do! loop (n + 1)
    loop 0)

let asyncReadInput =
   async {
       printf "> "
       let input = Console.ReadLine();
       return input

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() =
    Async.StartWithContinuations(asyncReadInput, (fun input ->
        let reply = agent.TryPostAndReply((fun replyChannel -> input, replyChannel), 1000)
        match reply with
        | None -> printfn "Timeout exceeded."
        | Some(reply) ->
            if (reply <> "Stop") then
                printfn "Reply: %s" reply
        (fun exn -> ()),
        (fun _ -> ()))

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

A sample session follows.

Mailbox Processor Test
Type some text and press Enter to submit a message.
Type 'Stop' to close the program.
> test1
Reply: Message number 0 was received. Message contents: test1
> test2
Reply: Message number 1 was received. Message contents: test2
> test3
Reply: Message number 2 was received. Message contents: test3
> test4
Reply: Message number 3 was received. Message contents: test4
> test5
Reply: Message number 4 was received. Message contents: test5
> test6
Timeout exceeded.
Press Enter to continue.

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

F# Runtime

Supported in: 2.0, 4.0


Supported in: 3




January 2011

Added code example.

Information enhancement.

Was this page helpful?
(1500 characters remaining)
Thank you for your feedback

Community Additions

© 2014 Microsoft