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

Waits for a message. This will consume the first message in arrival order.

Namespace/Module Path: Microsoft.FSharp.Control

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

// Signature:
member this.TryReceive : ?int -> Async<'Msg option>

// Usage:
mailboxProcessor.TryReceive ()
mailboxProcessor.TryReceive (timeout = timeout)


  • timeout
    Type: int

    An optional timeout in milliseconds. Defaults to -1 which corresponds to Infinite().

Return Value

An asynchronous computation (Async object) that returns the received message or None if the timeout is exceeded.


This method is for use within the body of the agent. Returns None if a timeout is given and the timeout is exceeded. This method is for use within the body of the agent. For each agent, at most one concurrent reader may be active, so no more than one concurrent call to Receive, TryReceive, Scan or TryScan may be active.


The following example shows how to use TryReceive. If a message is not received within 10 seconds, a timeout occurs and the message ID increments by 1.

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! opt = inbox.TryReceive(10000);
                match opt with
                | None -> do! loop(n + 1)
                | Some (message, replyChannel) ->
                    // The delay gets longer with each message, and eventually triggers a timeout.
                    if (message = "Stop") then
                        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 mutable isCompleted = false
while (not isCompleted) do
    printf "> "
    let input = Console.ReadLine()
    let reply = agent.PostAndReply(fun replyChannel -> input, replyChannel)
    if (reply <> "Stop") then
        printfn "Reply: %s" reply
        isCompleted <- true

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

A sample session follows. Notice that the message number 2 is skipped due to the timeout.

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 3 was received. Message contents: test3
> Stop
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

Version Information

F# Runtime

Supported in: 2.0, 4.0


Supported in: 3

See Also


Control.MailboxProcessor<'Msg> Class (F#)

Microsoft.FSharp.Control Namespace (F#)

Change History




January 2011

Added code example.

Information enhancement.