c# webcal:link in Outlook “internet calendar subscription" produce multiple instances of outlook calendars in outlook.exe

Question

I'm working with

• Windows 10 Professional 64-bit (21H1)
• MS Office 2013
• VS 2019 (v16.11.3)

I'm try to read a subscribed webcal - calendar with C#. Every time OpenSharedFolder(webCalString) is executed, the process starts with a small window saying “Connecting to Web server …”, then the Microsoft Outlook.exe user interface is opened and a new instance of this calendar is created under “My Calendars”. The problem does not occur if Outlook has been opened prior to starting my C# program.

My code is simple, it uses Microsoft.Office.Interop.Outlook. I added a reference to MS Office 15.0 Object library 2.7 (15.0.5363.1000):

using System;
using System.Windows.Forms;
using Outlook = Microsoft.Office.Interop.Outlook;

…

string webCalString = “webcal://koenigstein.mein-abfallkalender.de/ical.ics?sid=26035&cd=inline&ft=noalarm&fp=next_1000&wids=657,919,661,658,656,659,660,662,663&uid=46006&pwid=3ade10b890&cid=94”;

// initially set to NULL
Microsoft.Office.Interop.Outlook.Application oApp = null;
Microsoft.Office.Interop.Outlook.NameSpace mapiNamespace = null;
Microsoft.Office.Interop.Outlook.Folder CalendarFolderAbo = null; 

oApp = new Microsoft.Office.Interop.Outlook.Application();

mapiNamespace = oApp.GetNamespace(“MAPI”); ;

// these 2 lines are necessary, otherwise OpenSharedFolder(“webcal”) does not work (why???)
Microsoft.Office.Interop.Outlook.MAPIFolder CalendarFolder = null;
CalendarFolder = mapiNamespace.GetDefaultFolder(Microsoft.Office.Interop.Outlook.OlDefaultFolders.olFolderCalendar);

**// the following line opens outlook.exe, shows the User Interface of MS- Outlook, and always produces a new instance of this subscribed calendar under “My Calendars”**
CalendarFolderAbo = mapiNamespace.OpenSharedFolder(webCalString) as Outlook.Folder;

Answer 1

I found a workaround. The c# statement "OpenSharedFolder(webCalString)" creates new "Other Calendars" in MS Outlook. I did not find access to these private calendars with c#. However the following procedure allows to read internet web calendars:

1. Subscribe an internet web calendar in MS Outlook. This calendar appears in MS Outlook as "Shared Calendar"
2. The following code gets all shared subfolders in CalendarFolderWeb.Folders :

CalendarFolderWeb = mapiNamespace.GetDefaultFolder(Outlook.OlDefaultFolders.olFolderCalendar);

// Go through all subfolders

foreach (Outlook.MAPIFolder **subFolder** in CalendarFolderWeb.Folders) {

   outlookCalendarItemsWeb = subFolder.Items;

   outlookCalendarItemsWeb.IncludeRecurrences = true;

   ...

// now go thru the individual calendar items

   foreach (Outlook.AppointmentItem item in outlookCalendarItemsWeb) {
       string content = item.Subject;
   
       ...
   }
}