C# "Register COM interop" option is disabled

firstly verify your output type, he must be Class Library and not Console application.

Secondly if your problem persist, open your csproj and modify your node

<RegisterForComInterop>true</RegisterForComInterop>

I had the same problem, with the new Visual Studio 2019. The first choice for creating a Class Library is Class Library .Net framework and this will not work. You have to go down several lines to find Class Library (.NET framework) C# Windows library. This will work, provided that in project property you don't forget in Assembly Information to activate Make assembly copy visible in addition to register for com interop in build