At least with TypeScript, they've been using the official language server for a while. Sadly they still fall back onto their own JS-esque 'guess what this could

At least with TypeScript, they've been using the official language server for a while. Sadly they still fall back onto their own JS-esque 'guess what this could be' mode in some situations where vanilla TS would have just emitted an error that the respective module could not be found. For C# Rider supports Roslyn analyzers and code fixers as well, though I don't know with what performance impact (as ReSharper isn't based on Roslyn this results in all analysis work being done twice, which can be noticeable).