Mono

Custom fork: https://github.com/FlaxEngine/mono with custom features for C# assemblies hot-reloading at runtime without domain unload (more: https://flaxengine.com/blog/flax-facts-16-scripts-hot-reload/).

Some useful notes and tips for devs:

Use -monolog to print Mono logs to Flax logs
When working with mono fork set localRepoPath to local repo location in Source\Tools\Flax.Build\Deps\Dependencies\mono.cs
To update mono deps when developing/updating use .\Development\Scripts\Windows\CallBuildTool.bat -log -ReBuildDeps -verbose -depsToBuild=mono -platform=Windows, then build engine and run it
MONO_GC_DEBUG=check-remset-consistency - it will do additional checks at each collection to see if there are any missing write barriers
MONO_GC_DEBUG=nursery-canaries - it might catch some buffer overflows in case of problems in code.
MONO_GC_DEBUG=<log-level>:<log-file> - will print GC debug to the log file (eg. 4:sgen-gc).
Methods mono_custom_attrs_from_property and mono_custom_attrs_get_attr are internally cached
If C++ mono call a method in c# that will throw an error, error will be handled but, not completly. Calling relase domain will return random Access memory violation. First search for error in c# code. No workaround yet.