Question

While I do a program, sometimes I’ve got this doubt. I have been using List<T> but I haven’t used the others.

I’d like to know when each one is better to use, and under what circumstances.

Answer 1

I’m sure you can read the documentation by yourself. I’ll give a short summary here:

1. IEnumerable is an interface that exposes an enumerator over a collection. Implement this interface if want to be able to support iteration, for example inside a foreach loop.
2. Collection is an implementation of IEnumerable (therefore you can iterate over it) that is generally further extended by user-defined classes that want to have collection-like behavior (i.e. use Add, Remove, Contains, etc.). It can also be used “as-is”.
3. List is also an implementation of IEnumerable (therefore you can iterate over it) that is generally used “as-is” as a container for objects of a certain type. Uses a dynamically adjusting array in the background and is the generic equivalent of ArrayList.

Note that while IEnumerable can be both generic and non-generic, Collection and List can only be used as generic classes.