[trifurcate]

Mono as a runtime hasn't been relevant in almost a decade now (since the advent of .NET Core). "I can go into the compilers to find bugs" -> yeah, that's what Roslyn is. C# lets you control GC, marshalling, safety, calling convention, inlining, etc. for very fast hand-rolled managed or unmanaged code if you need it.

[neonsunset]

Mono is alive and well in https://github.com/dotnet/runtime/tree/main/src/mono. It serves Android, iOS, WASM and s390x targets, maybe some other too. It is generally much slower than CoreCLR, but besides supporting more platforms, has features that CoreCLR lacks like IL interpreter (technically, CoreCLR has one too but it has been broken for years and is never used).

Ideally, iOS's Mono usage will be eventually replaced with NativeAOT, but for now it's still being worked on, not in the least in iOS-targeting GUI frameworks like MAUI to provide better NAOT compatibility. In addition, NativeAOT's linker/trimmer is based on Mono.Linker and Mono.Cecil, so the project became part of .NET as was intended.

But you are right in a way, because the above is often confused with a separate, outdated Mono distribution that some people incorrectly keep insisting on installing on their Linux systems.

[trifurcate]

Yes, I wanted to mention WASM but honestly even talking about it is doing more of a disservice to modern .NET than not given the preconceptions about Mono.