For the dotnet world you can use and extend https://identityserver4.readthedocs.io/en/latest/
IdentityServer is a framework to roll your own IdP, it’s not fully functional out of the box like Keycloak.
I went with IdentityServer4 on a recent project over Keycloak and Gluu[1] for that reason and because it was in the same stack as the rest of our ecosystem.
[0] See comment from this thread https://news.ycombinator.com/item?id=22871756