Exchange Web Services Attachment load is slow

问题

I'm in the process of writing some code to download and process the attachments of email messages and then process them. The code is working as required in some cases, but still has some major problems.

Whenever the code loads the attachment into a file on the local disk, it takes a very long time to do so and often times out with the following exception as a result of the slow download:

A first chance exception of type 'Microsoft.Exchange.WebServices.Data.ServiceRequestException' occurred in Microsoft.Exchange.WebServices.dll

I may be wrong, but if the exchange server in question is on the same gigabit network as the server running the code and outlook has fast access to emails, attachments, etc then attachments should download considerably faster than they do and much more consistently. Here are some example of download/load times:

• 800KB Zip - 1m 4s
• 840KB Zip - 6m 18s
• 1.33MB Zip - 11m 23s
• 2.78MB Zip - 17m 3s

I have tried setting the EWS connection timeout setting to 300000ms instead of the default 100000ms to give the attachments more time to download and the number of exceptions has decreased slightly but the waiting time is now way too long.

The code does run in threads, no more than 8 at a time (10 being the throttling limit for EWS i believe), but i cant imagine that would make much of a difference. (It hasn't done when I've been testing single emails at a time).

Here is the threaded code that downloads the attachments (some un-related bits removed for simplicity):

        Dim strMessageFolder As String

        ' Prepare the directory where this emails attachments will be stored
        strMessageFolder = g_strFolder_Temp & strMessageID & "\"

        ' Create a folder to store the attachments for this email
        Call FileSystem_CreateFolder(strMessageFolder, True)

        ' Process the emails attachments
        For Each emailAttachment In emailMessage.Attachments
            Dim fileattach As FileAttachment
            'Dim fileattachStream As FileStream
            Dim strAttachmentFile As String

            ' Prepare for the downloading of the attachment
            fileattach = emailAttachment
            blnTryFailed = False
            intAttempts = 0
            strAttachmentFile = strMessageFolder & fileattach.Name

            ' Handle up to 3 download attempts
            Do
                Try
                    ' Try to download the attachment - Method 1
                    fileattach.Load(strAttachmentFile)

                    ' Try to download the attachment - Method 2
                    'fileattachStream = New FileStream(strAttachmentFile, FileMode.OpenOrCreate, FileAccess.ReadWrite)
                    'fileattach.Load(fileattachStream)
                    'fileattachStream.Close()
                    'fileattachStream.Dispose()

                    blnTryFailed = False

                Catch ex As Exception
                    blnTryFailed = True

                    ' Ensure the failed download is deleted
                    Call FileSystem_DeleteFile(strAttachmentFile)

                    intAttempts += 1

                End Try

            Loop While blnTryFailed And intAttempts < 3

            ' If the attachment download was unsuccessful then we cannot process the current email
            If blnTryFailed = True Then
                emailMessage.IsRead = False
                'message.Subject = message.Subject & " - Attachment download failed, skipped"
                Try
                    emailMessage.Update(ConflictResolutionMode.AutoResolve)
                Catch ex As Exception
                    Call Logging_Add("Unable to mark email as skipped", strMessageID, "Debug")
                End Try
                Exit Sub
            End If

As mentioned previously, im aware of the Exchange throttling but cannot find anything related to the speed at which attachments are downloaded. So my question is what could possibly be causing such slow download speeds?

回答1:

I have the same issue with my app. The problem was caused by default EWS setting which writes to Console all HttpRequest and HttpResponse messages between EWS and application. Turning off TraceFlags was blessing. My code in c#:

ExchangeService service = new ExchangeService(ExchangeVersion.Exchange2010_SP2);
service.TraceFlags = TraceFlags.None;

来源：https://stackoverflow.com/questions/10091745/exchange-web-services-attachment-load-is-slow