It's Always best to use the lowest base type possible. This gives the implementer of your interface, or consumer of your method, the opportunity to use whatever they like behind the scenes. IList is an interface so you güç inherit another class and still implement IList while inheriting List prevents https://marcojwemt.blogzet.com/en-kuralları-of-c-ilist-neden-kullanmalıyız-42782863