WCF Restful Service：异步服务操作

Question

我有一个同步端点，该端点允许您下载文件，并且希望它变得异步。查看以下文档（https://docs.microsoft.com/en-us/dotnet/framework/wcf/how-to-implement-an-asynchronous-service-operation），我以这种方式修改了代码，但是从测试中我发现，文件的恢复（由“ New FileStream”确定）仍然是同步的。

    <OperationContractAttribute(AsyncPattern:=True)>
    <WebGet(UriTemplate:="RecuperaPacchettoFinitoAsync?l={sLoginName}&f={sFilename}&g={sGuid}&tp={sTipoProdotto}&v={sVersione}&bi={sBuildIniziale}&bf={sBuildFinale}&bd={bDocumentazione}&t={token}")>
    Function BeginDownloadFileAsync(sLoginName As String, sFilename As String, sGuid As String, sTipoProdotto As String, sversione As String, sBuildIniziale As String, sBuildFinale As String, bDocumentazione As Boolean, token As String, callback As AsyncCallback, asyncState As Object) As IAsyncResult
    Function EndDownloadFileAsync(result As IAsyncResult) As Stream

...

Private Function BeginDownloadFileAsync(sloginName As String, sFilename As String, sGuid As String, sTipoProdotto As String, sVersione As String, sBuildIniziale As String, sBuildFinale As String, bDocumentazione As Boolean, sToken As String, callback As AsyncCallback, asyncState As Object) As IAsyncResult Implements ILiveUpdateUIWS.BeginDownloadFileAsync
        If (LU_Helper.ValidaToken(sToken, DB_CONNECTION_STRING) = True) Then
            Try
                Dim sFilenameInCache As String = String.Format("{0}{1}_{2}_{3}.zip", sTipoProdotto, sVersione, sBuildIniziale.PadLeft(10, "0"c), sBuildFinale.PadLeft(10, "0"c))
                If sTipoProdotto = "SampleProduct" Then
                    sFilenameInCache = sFilename
                End If

                If (My.Computer.FileSystem.FileExists(String.Format("{0}\{1}", DESTINATION_PATH_PACKAGES_COMPLETED, sFilenameInCache)) AndAlso bDocumentazione = False) Then
                    Dim downloadFilePath As String = String.Format("{0}\{1}", DESTINATION_PATH_PACKAGES_COMPLETED, sFilenameInCache)
                    WebOperationContext.Current.OutgoingResponse.ContentType = "application/zip"
                    Dim fstream = New FileStream(downloadFilePath, FileMode.Open)
                    Return New CompletedAsyncResult(Of Stream)(fstream)

                Else
                    Dim downloadFilePath As String = String.Format("{0}\{1}\{2}\{3}", DESTINATION_PATH_ZIP_FILE, sloginName, sGuid, sFilename)
                    WebOperationContext.Current.OutgoingResponse.ContentType = "application/zip"
                    Dim fstream = New FileStream(downloadFilePath, FileMode.Open)
                    Return New CompletedAsyncResult(Of Stream)(fstream)

                End If

            Catch ex As Exception
                TraceLog.WriteToErrorLog(ex.Message, Environment.StackTrace, "Error")
                WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.InternalServerError
                Return Nothing
            End Try
        Else
            WebOperationContext.Current.OutgoingResponse.StatusCode = HttpStatusCode.Forbidden
            Return Nothing
        End If
    End Function

    Private Function EndDownloadFileAsync(r As IAsyncResult) As Stream Implements ILiveUpdateUIWS.EndDownloadFileAsync
        Dim result As CompletedAsyncResult(Of Stream) = TryCast(r, CompletedAsyncResult(Of Stream))
        Return result.Data
    End Function

...

Class CompletedAsyncResult(Of T)
    Implements IAsyncResult

    Private datalocal As T

    Public Sub New(ByVal data As T)
        datalocal = data
    End Sub

    Public ReadOnly Property Data As T
        Get
            Return datalocal
        End Get
    End Property

    Public ReadOnly Property AsyncState As Object Implements IAsyncResult.AsyncState
        Get
            Return CObj(datalocal)
        End Get
    End Property

    Public ReadOnly Property AsyncWaitHandle As WaitHandle Implements IAsyncResult.AsyncWaitHandle
        Get
            Throw New Exception("The method or operation is not implemented.")
        End Get
    End Property

    Public ReadOnly Property CompletedSynchronously As Boolean Implements IAsyncResult.CompletedSynchronously
        Get
            Return True
        End Get
    End Property

    Public ReadOnly Property IsCompleted As Boolean Implements IAsyncResult.IsCompleted
        Get
            Return True
        End Get
    End Property
End Class