The basic C# libraries are full of undocumented surprising behavior, and surprising behavior that is documented only on thematic docs and not on the function docs. And most of the ecosystem follow the convention.
There are many people that insist the .Net environment is well documented because there is documentation for every single item, but that documentation is far from complete.
There are many people that insist the .Net environment is well documented because there is documentation for every single item, but that documentation is far from complete.