异步计算不能捕获OperationCancelledException

Question

我正在尝试向URL发出异步web请求，如果请求花费太长时间，该请求将返回。我使用F#异步工作流和System.Net.Http库来完成这个任务。

但是，我无法捕获由System.Net.Http库在async工作流中引发的Task/OperationCancelledExceptions。相反，异常是在Async.RunSynchronously方法中引发的，正如您在这个堆栈跟踪中看到的那样：

> System.OperationCanceledException: The operation was canceled.    at
> Microsoft.FSharp.Control.AsyncBuilderImpl.commit[a](Result`1 res)   
> at
> Microsoft.FSharp.Control.CancellationTokenOps.RunSynchronously[a](CancellationToken
> token, FSharpAsync`1 computation, FSharpOption`1 timeout)    at
> Microsoft.FSharp.Control.FSharpAsync.RunSynchronously[T](FSharpAsync`1
> computation, FSharpOption`1 timeout, FSharpOption`1 cancellationToken)
> at <StartupCode$FSI_0004>.$FSI_0004.main@()

守则：

#r "System.Net.Http"

open System.Net.Http
open System

let readGoogle () = async {
    try
        let request = new HttpRequestMessage(HttpMethod.Get, "https://google.co.uk")
        let client = new HttpClient()
        client.Timeout <- TimeSpan.FromSeconds(0.01) //intentionally low to always fail in this example
        let! response = client.SendAsync(request, HttpCompletionOption.ResponseContentRead) |> Async.AwaitTask 
        return Some response
    with 
        | ex ->
            //is never called
            printfn "TIMED OUT" 
            return None
}

//exception is raised here
readGoogle ()
    |> Async.RunSynchronously
    |> ignore

Answer 1

取消总是与错误不同。在您的情况下，您可以覆盖AwaitTask的默认行为，如果任务被取消，则调用"cancel延续“，并以不同的方式处理它：

let readGoogle () = async {
    try
        let request = new HttpRequestMessage(HttpMethod.Get, "https://google.co.uk")
        let client = new HttpClient()
        client.Timeout <- TimeSpan.FromSeconds(0.01) //intentionally low to always fail in this example
        return! ( 
            let t = client.SendAsync(request, HttpCompletionOption.ResponseContentRead)
            Async.FromContinuations(fun (s, e, _) ->
                t.ContinueWith(fun (t: Task<_>) -> 
                    // if task is cancelled treat it as timeout and process on success path
                    if t.IsCanceled then s(None)
                    elif t.IsFaulted then e(t.Exception)
                    else s(Some t.Result)
                )
                |> ignore
            )
        )
    with 
        | ex ->
            //is never called
            printfn "TIMED OUT" 
            return None
}