MailboxProcessor<T> from C#

Question

Have you tried to use a MailboxProcessor of T from C#? Could you post sample code?

How do you start a new one, post messages to it, and how do you process them?

Answer 1

While you can use MailboxProcessor<T> directly from C# (using the C# async extension) as pointed out in my other answer, this isn't really a good thing to do - I wrote that mainly for curiosity.

The MailboxProcessor<T> type was designed to be used from F#, so it doesn't fit well with the C# programming model. You could probably implement similar API for C#, but it wouldn't be that nice (certainly not in C# 4.0). The TPL DataFlow library (CTP) provides similar design for the futrue version of C#.

Currently, the best thing to do is to implement the agent using MailboxProcessor<T> in F# and make it friendly to C# usage by using Task. This way, you can implement the core parts of agents in F# (using tail-recursion and async workflows) and then compose & use them from C#.

I know this may not directly answer your question, but I think it's worth giving an example - because this is really the only reasonable way to combine F# agents (MailboxProcessor) with C#. I wrote a simple "chat room" demo recently, so here is an example:

type internal ChatMessage = 
  | GetContent of AsyncReplyChannel<string>
  | SendMessage of string

type ChatRoom() = 
  let agent = Agent.Start(fun agent -> 
    let rec loop messages = async {
      // Pick next message from the mailbox
      let! msg = agent.Receive()
      match msg with 
      | SendMessage msg -> 
          // Add message to the list & continue
          let msg = XElement(XName.Get("li"), msg)
          return! loop (msg :: messages)

      | GetContent reply -> 
          // Generate HTML with messages
          let html = XElement(XName.Get("ul"), messages)
          // Send it back as the reply
          reply.Reply(html.ToString())
          return! loop messages }
    loop [] )
  member x.SendMessage(msg) = agent.Post(SendMessage msg)
  member x.AsyncGetContent() = agent.PostAndAsyncReply(GetContent) 
  member x.GetContent() = agent.PostAndReply(GetContent)

So far, this is just a standard F# agent. Now, the interesting bits are the following two methods that expose GetContent as an asynchronous method usable from C#. The method returns Task object, which can be used in the usual way from C#:

  member x.GetContentAsync() = 
    Async.StartAsTask(agent.PostAndAsyncReply(GetContent))

  member x.GetContentAsync(cancellationToken) = 
    Async.StartAsTask
     ( agent.PostAndAsyncReply(GetContent), 
       cancellationToken = cancellationToken )

This will be reasonably usable from C# 4.0 (using the standard methods such as Task.WaitAll etc.) and it will be even nicer in the next version of C# when you'll be able to use the C# await keyword to work with tasks.