The package "Hafner.Tools.Settable" (formerly "Hafner.Tools.SetTracker") provides a tiny helper type of the same name that can be used to differentiate between whether a parameter/property was set to null or was not set at all. Useful for filters (null not set = do not apply any filter for that property, null set = return all records that have null in that column) or UpdateRecord methods (null not set = don't touch that value, null set = change the value to null) etc. Also helpful in data transfer objects (DTOs) that are declared with nullable reference types enabled.
$ dotnet add package Hafner.Tools.SettableNo README available.