F＃中的異步等待

Question

我將本實驗中的一些C＃重寫為F＃： https ： //github.com/Microsoft/TechnicalCommunityContent/tree/master/IoT/Azure%20Stream%20Analytics/Session%202%20-%20Hands%20On

我正在練習6，＃17 - 創建SimpleEventProcessor類型。
我想實現CloseAsync方法

C＃

async Task IEventProcessor.CloseAsync(PartitionContext context, CloseReason reason)
    {
        Debug.WriteLine("Processor Shutting Down. Partition '{0}', Reason: '{1}'.", context.Lease.PartitionId, reason);
        if (reason == CloseReason.Shutdown)
        {
            await context.CheckpointAsync();
        }
    }

我開始是這樣的：

member this.CloseAsync(context, reason) = 
    Debug.WriteLine("Processor Shutting Down. Partition '{0}', Reason: '{1}'.", context.Lease.PartitionId, reason)
    match reason with 
    | CloseReason.Shutdown -> await context.CheckpointAsync()
    | _ -> ()

但我有兩個問題：

1. 如何在F＃世界中返回等待？
2. 如何返回NOT情況 - > C＃只是忽略了這種可能性。

Answer 1

1. 如果值的類型為Async<'T> ，則可以在沒有任何關鍵字的情況下返回它。 如果它具有類型Task或Task<'T> ，則可以執行|> Async.AwaitTask 。

2. 您可以返回async { return () } 。

所以你得到這個：

member this.CloseAsync(context, reason) = 
    Debug.WriteLine("Processor Shutting Down. Partition '{0}', Reason: '{1}'.", context.Lease.PartitionId, reason)
    match reason with 
    | CloseReason.Shutdown -> context.CheckpointAsync() |> Async.AwaitTask
    | _ -> async { return () }

另一種可能性是將整個塊放在async工作流中，並使用return! 為1並return 2：

member this.CloseAsync(context, reason) = 
    async {
        Debug.WriteLine("Processor Shutting Down. Partition '{0}', Reason: '{1}'.", context.Lease.PartitionId, reason)
        match reason with 
        | CloseReason.Shutdown -> return! context.CheckpointAsync() |> Async.AwaitTask
        | _ -> return ()
    }

事實上，使用異步工作流允許您刪除()情況類似於C＃：

member this.CloseAsync(context, reason) = 
    async {
        Debug.WriteLine("Processor Shutting Down. Partition '{0}', Reason: '{1}'.", context.Lease.PartitionId, reason)
        if reason = CloseReason.Shutdown then
            return! context.CheckpointAsync() |> Async.AwaitTask
    }