I can't trigger Microsoft.Office.Interop.Outlook.dll via .NET

Question

I have installed Visual Studio Tools for Office (VSTO) so can use Microsoft.Office.Interop.Outlook.dll for my .Net applications. However, I get the error below. I think it is related to registry keys but I am not sure. Microsoft.Office.Interop.Outlook.dll exists but the registry is looking for it somewhere else maybe? I used to use this .dll library on my previous computer w/o problem. By then, I did not install VSTO but it was something similar. Do you have any idea how can I fix the issue?

3rd line raises the exception below on the screenshot

using Microsoft.Office.Interop.Outlook;

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

Microsoft.Office.Interop.Outlook.MailItem oMsg = (Microsoft.Office.Interop.Outlook.MailItem)oApp.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);

I found these Unable to cast COM object of type 'Microsoft.Office.Interop.Outlook.ApplicationClass' {00063001-0000-0000-C000-000000000046} HRESULT: 0x80040155

https://social.msdn.microsoft.com/Forums/vstudio/en-US/ac50fa41-8d47-4fa9-81a3-914f262676af/0x80029c4a-typeecantloadlibrary?forum=vsto

but they didn't help

Answer 1

I backed up and deleted both Win64 Keys under

00062FFF-… 2DF8D04C…

Then it worked.

Here is my guess:

We upgraded the latest 2021 Office (64 bit) and downgraded back to 2016 Office (32 bit) and I believe this was the root of the problem.

I think VS was trying to use 64-bit Outlook but of course it was not there. So, it was showing broken references (yellow triangle sign) on references of

Microsoft.Office.Core Microsoft.Office.Interop.Outlook

I tried to browse and connect the library (i.e., right-click References > Add Reference > Browse) directly but had no luck. I think VS was still referring to 64-bit Office/Outlook

When I removed the Win64 keys VS was forced to use 32-bit Office which exists and it worked fine.

It took a while to figure this out I hope this answer helps other users. By the way, I don't know if deleting these registries will cause any problem when we upgrade later Office version again.

Answer 2

There is no need to use VSTO if you automate Outlook from your .Net based application. VSTO stands for Visual Studio Tools for Office which is designed for developing Office COM add-ins.

It seems you have got some problems with Windows registry keys messed up. I'd suggest repairing MS Office to recover missed keys or try looking in the windows registry keys manually like described in the articles mentioned in your post. There is no universal fix for that.