相关疑难解决方法(0)

在调用恰好为空的嵌套异步时，我偶然发现了一个问题。引发了异常，但无法使用异步工作流提供的任何正常异常处理方法捕获它。

以下是重现问题的简单测试：

[<Test>]
let ``Nested async is null with try-with``() = 

    let g(): Async<unit> = Unchecked.defaultof<Async<unit>>

    let f = async {
            try
                do! g()
            with e ->  
                printf "%A" e
    }

    f |> Async.RunSynchronously |> ignore

这导致以下异常：

System.NullReferenceException : Object reference not set to an instance of an object.
at Microsoft.FSharp.Control.AsyncBuilderImpl.bindA@714.Invoke(AsyncParams`1 args)
at <StartupCode$FSharp-Core>.$Control.loop@413-40(Trampoline this, FSharpFunc`2 action)
at Microsoft.FSharp.Control.Trampoline.ExecuteAction(FSharpFunc`2 firstAction)
at Microsoft.FSharp.Control.TrampolineHolder.Protect(FSharpFunc`2 firstAction)
at Microsoft.FSharp.Control.AsyncBuilderImpl.startAsync(CancellationToken cancellationToken,     FSharpFunc`2 cont, FSharpFunc`2 econt, FSharpFunc`2 ccont, FSharpAsync`1 p)
at Microsoft.FSharp.Control.CancellationTokenOps.starter@1121-1.Invoke(CancellationToken     cancellationToken, FSharpFunc`2 cont, FSharpFunc`2 …