asp.net - 4.5 中的异步 : Cancellation and Timeout

Question

我在 HTTPHandler 中有一个长时间运行的进程，我使用 Async Await .net 4.5 异步执行该进程

这完全符合预期......

如何添加超时，以便在进程花费太长时间时返回字符串“timeout”？

Protected Async Sub button1_Click(sender As Object, e As System.EventArgs)
    Dim asyncHandler = New AsyncHandler
    Await asyncHandler.ProcessRequestAsync(HttpContext.Current)
    Response.Write(asyncHandler.Result)
End Sub

Public Class AsyncHandler
    Inherits HttpTaskAsyncHandler

    Public Property Result As String

    Public Async Function ProcessRequestAsync(context As HttpContext) As Task
        Me.Result = Await DoLongRunningProcessAsync(context)
    End Function

    Private Function DoLongRunningProcessAsync(context As HttpContext) As Task(Of String)

        'TODO: add a timeout so that if this takes too long we return "timeout":

        Return Task.Run(Of String)(Function() DoLongRunningProcess(context))

    End Function

    Private Function DoLongRunningProcess(context As HttpContext) As String
        'perform long running process....

        Return "success"
    End Function

End Class

Answer 1

这是我最终实现的:

创建了一个超时值为 x 毫秒的 CancellationToken

添加了 try catch 来捕获错误:OperationCanceledException

Public Async Function ProcessRequestAsync(context As HttpContext) As Task
        Dim cts As CancellationTokenSource = New CancellationTokenSource(30000) 'eg: 30 seconds
        Try
            Me.ResultCode = Await DoLongRunningProcessAsync(HttpContext.Current, cts.Token)
        Catch ex As OperationCanceledException
            Me.ResultCode = "timeout"
        End Try
End Function

将 CancellationToken 传递给 DoLongRunningProcessAsync 方法的签名

通过调用 ThrowIfCancellationRequested 检测超时

Private Function DoLongRunningProcessAsync(context As HttpContext, ct As CancellationToken) As Task(Of String)
        Return Await Task.Run(Of String)(
            Function()
                Dim resultCode As String = DoLongRunningProcess(context)

                'detect timeout:
                ct.ThrowIfCancellationRequested()

                Return resultCode
            End Function)
End Function

这是我发现确实有帮助的一篇很棒的文章:

Async in 4.5: Enabling Progress and Cancellation in Async APIs