Roslyn Analyzers are extensions that analyze source code and report violations. Some analyzers are built-into VS (like the IDE analyzers that report style issues) and some are third party ones which can be installed (like StyleCopyAnalyzers, FxCopAnalyzers, etc.). Analyzers can be configurated using file(s) to allow end users to control the behavior of each analyzer and severity of reported diagnostics. This may be via an editorconfig or a ruleset file or even completely custom format specific to the third party analyzer package.