如何在Elmish/F#中捕获后台线程的打印错误

我在F#中实现了一个使用STA线程的打印函数：

let PrintProgressNoteAsync (encounterid:int)  = 
              async {
                    let mutable printmsg = null
                    let thread = Thread(fun () -> printmsg <- PrintProgressNote encounterid);
                    thread.SetApartmentState(ApartmentState.STA); 
                    thread.Start(); 
                    thread.Join(); 
                    return printmsg
              }

打印成功时，PrintProgressNote会返回字符串：

sprintf "Progress note printed to %s" printername

但打印操作容易失败，需要将后台线程中PrintProgressNote内的打印机错误向上传递，最终由Elmish捕获并处理，让Cmd.Async.either Print encounterid OnPrintSuccess OnPrintError能够接收成功时的字符串和错误时的exn。

原代码的核心问题是：STA线程内部抛出的异常无法自动传递到外层async块——独立线程的未处理异常会直接终止线程，外层的thread.Join()无法感知到错误。需要手动捕获线程内的异常，再传递到外层async块中抛出，让Elmish的错误处理逻辑能捕获到。

方式一：直接捕获并传递异常 #

在线程函数内捕获异常并保存，外层async块根据情况返回结果或抛出异常：

let PrintProgressNoteAsync (encounterid:int) = 
    async {
        let mutable error = None
        let mutable printMsg = null
        // 定义STA线程执行逻辑，包含异常捕获
        let thread = Thread(fun () ->
            try
                printMsg <- PrintProgressNote encounterid
            with
            | ex -> error <- Some ex
        )
        thread.SetApartmentState(ApartmentState.STA)
        thread.Start()
        thread.Join()
        
        // 处理结果：有异常则抛出，无异常则返回消息
        match error with
        | Some ex -> return! async.Raise ex
        | None -> return printMsg
    }

方式二：用结果类型封装（语义更清晰） #

先定义明确的结果类型区分成功/失败，再在线程内填充结果，外层统一处理：

// 定义结果类型，封装成功消息或异常
type PrintOutcome =
    | PrintSuccess of string
    | PrintFailure of exn

let PrintProgressNoteAsync (encounterid:int) = 
    async {
        let mutable outcome = PrintFailure (Exception("线程未执行"))
        let thread = Thread(fun () ->
            try
                let msg = PrintProgressNote encounterid
                outcome <- PrintSuccess msg
            with
            | ex -> outcome <- PrintFailure ex
        )
        thread.SetApartmentState(ApartmentState.STA)
        thread.Start()
        thread.Join()
        
        // 根据结果类型返回或抛出异常
        match outcome with
        | PrintSuccess msg -> return msg
        | PrintFailure ex -> return! async.Raise ex
    }

效果说明 #

修改后，当PrintProgressNote成功执行时，PrintProgressNoteAsync返回打印消息，Elmish的OnPrintSuccess会被触发；当打印抛出异常时，异常会被传递到外层async块并抛出，Cmd.Async.either会捕获到异常，调用OnPrintError处理。

内容的提问来源于stack exchange，提问作者Alan Wayne