What do you like to use?
1. Elixir: Pros: Erlang threading model is clearly the best I've come across, expressive syntax, strong data structures and algorithms in standard library. Cons: Slow for some things, anemic standard library for common problem domains, small community.
2. Python: Pros: Strong standard library for common problem domains, supplemented by a strong community set of libraries. Generally intuitive. Cons: Slow for some things, poor threading, inconsistencies in syntax, general dominance of configuration-oriented programming frameworks rather than libraries in the community, types are just not quite strong enough.
3. C#: Pros: strong type system, speed. Cons: dependency injection hell, MS walled garden.
I'd like to add Rust and/or OCaml, to this list, but I don't know either well enough to be confident.
.NET Core is FOSS though?
My guess is that MS proprietary tooling still sucks out the air from FOSS tools, but I can't speak from experience there so I'll just admit I don't know how the ecosystem has changed since I worked in it.
It's nearly impossible to find out which of the umpteen repositories contains the source for the class you're trying to understand, and MSDN won't be of much help. Much of the standard library also has a bunch of decoy source files that only contain signatures and/or mocks, so it's a pain to find the real version.
OmniSharp has a "go-to-source" option, but you only get the real source if it's in the same solution. Otherwise you only get signatures. Try giving Metals[0] or Rust-Analyzer[1] a shot, this stuff is night-and-day.
The official debugger (vsdbg) only works in VS and VSCode (and explicitly not in FOSS builds of VSCode). There is netcoredbg[2], but I still haven't figured out how to convince Emacs to use it. This wouldn't be as much of a problem if the culture wasn't so hostile to printf debugging (for example, ~nobody bothers to implement helpful ToStrings).
There is a heavy cultural bias towards MS libraries, regardless of their merits (ASP.Net Core, DI hell, EFCore, etc). See [3] for an.. interesting example.
OmniSharp has a habit of giving up randomly, giving useless error messages, and most GH issues consist of people repeating "+1", until there is a one-liner about how an update fixed it. Root cause analysis, what's that?
[0]: https://scalameta.org/metals/
[1]: https://rust-analyzer.github.io/
[2]: https://github.com/Samsung/netcoredbg
[3]: https://old.reddit.com/r/csharp/comments/9kha39/does_anybody...
I have to admit I almost never run into this issue because because I don't really care what the source of the standard library does, the docs have always been sufficient for me. You could try https://source.dot.net/ though.
In the very rare case I do want to see the source of some 3rd party code I use dnSpy. It isn't as nicely integrated, but it's a very nice decompiler and debugger.