I have a class that is defined in a file in Project A. I want to use that
file in Project B, but I do NOT want to copy the file to Project B. I want
only to add a link to the file from Project B. I figured out how to do this
a few weeks ago, and now I've forgotten how. When it is linked, not copied,
the file will show in the solution explorer in Project B with a shortcut
arrow.

TIA

Ok, never mind, I found it again. You have to do it from the top menu File
=> Project => Add existing item, AND you have to click the almost-invisible
down arrow on the "Open" button in the file add dialog, and then choose
"Link to file".

It's almost an Easter egg.