This is a proof-of-concept for making a COM-based Outlook addin using .NET 5, no VSTO required.
This does not currently work due to bugs in WinForms. See dotnet/winforms#4370
This project is intended to be used with a custom patched build of WinForms. You'll likely need to update the path in the project file for the WinForms references.
After building, you need to manually register the COM host. Open an Admin command propmt in the output folder and run regsvr32 OutlookAddinNet5.comhost.dll
You need to add some registry keys to make Outlook load the addin, use the file registeraddin.reg to do this.
When starting Outlook, you should see two buttons on the ribbon.
Button 1 will just show a message box.
Button 2 is intended to open a custom task pane hosting a WinForms UserControl. This functionality is currently bropken