Looping through inbox items on timer event is really slow...?

Question

Hi,
I have an issue with a email sync event that I am running.  Essentially, I am given a date (the received date of the last email synced) via another process and then need to return all the emails in the user's inbox the where receivedDate > this given
date.
I have a timer object than triggers my sync event every 15 mins.  I also have a button that allows the user to manually activate the sync - this basically changes the timer to 3 secs and so it fires the sync event.
If the sync event is fired manually as per above, it loops through everything quite quickly (~12,000 emails in 30 secs) and I can stored the sender/recipient and received time data I require from these in a list of objects, which I sync later on in a background
process.  During this loop the main UI freezes up, which is fine since I show a progress bar so the user knows how long they need to wait, and a large sync like this should only occur on their first sync theoretically.  If the first sync event is
fired without the manual input - so still called from the same timer event, but just on the normal 15 min event, not the shortened 3 sec one - then it runs differently.  The sync takes a lot lot longer, and the UI doesn't freeze up.
I presume this might be because it is fired slightly differently and does not hog up the main thread, but rather shares it with normal Outlook operations.  Maybe the operations are being run on a background thread and then keep having to jump into the
main thread to access the email items...?  Either way, I'm not quite sure how this is happening since the only difference is the user pressing a button that changes the timer.interval.  The code that is run is the same, and the button press doesn't
trigger the actual sync itself.
Is there a way I can force this first sync to always run solely in the main thread, it can freeze up Outlook and show the progress bar until it is done?  Ideally any subsequent syncs would then be run with no progress bar and no freezing up of the UI.
Many thanks,
Tom

Answer 1

Hello Tom,
> Maybe the operations are being run on a background thread and then keep having to jump into the main thread to access the email items...?
You shouldn't use another threads when dealing with the Outlook object model. Office applications use the single threaded apartment (STA) model and don't support multithreading. All calls made from another threads are marshalled by Outlook to the main
thread. However, you can use a low-level code (Extended MAPI) to access the data from secondary threads. For example, you can use Redemption which is based on Extended MAPI.
>  it loops through everything quite quickly (~12,000 emails in 30 secs)
Instead of looping over all items in the folder I'd recommend using the
Find/FindNext
or
Restrict methods of the Items class. You can read more about them in the following articles:
How To: Use Find and FindNext methods to retrieve Outlook mail items from a folder (C#, VB.NET)
How To: Use Restrict method to retrieve Outlook mail items from a folder
You can use the System.Windows.Forms.Timer class which uses the main thread for invoking the Tick event. The .NET
Framework Class Library provides three different timer classes: System.Windows.Forms.Timer, System.Timers.Timer, and System.Threading.Timer. Each of these classes has been designed and optimized for use in different situations. The Comparing
the Timer Classes in the .NET Framework Class Library article examines the three timer classes and helps you gain an understanding of how and when each class should be used.
Also you may find the
AdvancedSearch method of the Application class helpful. Pay special attention to the fact that the search is performed in another thread. You don’t need to run another thread manually since the
AdvancedSearch method runs it automatically in the background. See
Advanced search in Outlook programmatically: C#, VB.NET for more information.