ICS file rendering raw HTML when opened in Microsoft Outlook 2016 but not in MS Outlook 2013

Question

I have ICS file which is created from T4 template as follows.

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 11.0 MIMEDIR//EN
VERSION:2.0
BEGIN:VEVENT
DTSTART:20190129T013000Z
DTend:20190129T023000Z
SUMMARY;ENCODING=QUOTED-PRINTABLE: Hasitha Dananjaya
LOCATION;ENCODING=QUOTED-PRINTABLE: 
DESCRIPTION;CONTENT-TYPE:text/html:ENCODING=QUOTED-PRINTABLE:<p>test test</p><p>&nbsp;</p><p>&nbsp;</p><p>test line 2</p>=0D=0A=0D=0A
X-ALT-DESC;FMTTYPE=text/html:<p>test test</p><p>&nbsp;</p><p>&nbsp;</p><p>test line 2</p>
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
CATEGORIES:Business
PRIORITY:3
END:VEVENT
END:VCALENDAR

When its opened from MS Outlook 2016 it shows raw html values as follows.

But same file opened in MS Outlook 2013

I did make sure both of the clients are running HTML mode.

I'm just trying to figure out what could be the root cause for this. Any help is appreciated.

Answer 1

Do reverse engineering, create the file from Outlook 2016. Save the ICS file and open and view the content.

Then use this content to replace the logic how it generates.

Answer 2

Actual issue was with DESCRIPTION and X-ALT-DESC fields.

According to RFC2445 DESCRIPTION is to specify the plain text body of an appointment and X-ALT-DESC is there to provide alternate format for the DESCRIPTION property with HTML.

RFC - DESCRIPTION

RFC - X-ALT-DESC

Somehow MS OUTLOOK 2013 reads X-ALT-DESC by default where OUTLOOK 2016 reads DESCRIPTION by default. 2016 falls back to X-ALT-DESC only when DESCRIPTION is not there.

Therefore, Fix was to keep ICS file as follows.

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 11.0 MIMEDIR//EN
VERSION:2.0
BEGIN:VEVENT
DTSTART:20190201T013000Z
DTend:20190201T023000Z
SUMMARY;ENCODING=QUOTED-PRINTABLE: Hasitha Dananjaya
LOCATION;ENCODING=QUOTED-PRINTABLE: 
DESCRIPTION:\naaaaaaaaaaaaa\naaaaaaaaaaaaaaaaaaaaaaa\naaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\nbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
X-ALT-DESC;FMTTYPE=text/html:<p>aaaaaaaaaaaaa</p><p>aaaaaaaaaaaaaaaaaaaaaaa</p><p>aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</p><p>bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb</p>
X-MICROSOFT-CDO-BUSYSTATUS:BUSY
CATEGORIES:Business
PRIORITY:3
END:VEVENT
END:VCALENDAR