I heard that it is important to use the lowest class when passing parameters to methods. Why is this? Also where can I find more information on what a class hierarchy is? I would like to know what IEnumerable belongs to and so on.
If you use IEnumerable<T>as a parameter type, then you can pass any type that implements this interface. This includes List<T>, Stack<T>, Queue<T>etc.
IEnumerable<T>
List<T>
Stack<T>
Queue<T>
It also includes various anonymous types, which can be the result of a LINQ query, as well as a very important one IQueryable<T>.
IQueryable<T>
"low-level", . .
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API , IList<T> , , , , IEnumerable<T>, ReadOnlyCollection. , .Add/.Remove ..
IList<T>
, , IEnumerable, /, IList.
: List<T>, IList<T>. , List - , - , IList - , , .
, , : , , List<T>?
, - LinkedList<T> SortedList<T>, , , , , .
LinkedList<T>
SortedList<T>
, .
The idea is to maximize the flexibility of your function. If you need a List <T>, then callers must have one to go through. If they do not have one convenient, they will have to create it, and it is expensive. If you require IEnumerable <T>, on the other hand, they can be passed to any collection.
The best place to learn about class heiarchy in .NET is MSDN.