Question

I can write my code in both the ways to achieve my objective, but wondering which one would be the best practice to adhere?

Collection<MyClass> obj = new Collection<MyClass>();

or

IEnumerable<MyClass> obj = new Collection<MyClass>();

Answer 1

In this case, IEnumerable<MyClass> obj = new Collection<MyClass>() makes no sense at all because you have an empty collection that you cannot add items to (because IEnumerable does not allow it). How is that useful?

Other than that, if we ‘re talking about a local variable that does not get exposed to code outside a method, then the only thing that matters is convenience and clarity. You can max out both of these by using an equivalent of the first form:

var obj = new Collection<MyClass>();

If the object is exposed to outside code (e.g. through a property, or if it’s returned by a method) then you should choose the type that makes the best sense given the public interface of your class. This means that as a rule of thumb, you should use the least specialized type possible, and always prefer interfaces.

So if you were exposing obj you should use IEnumerable if the collection is not meant to be modified by users of your code. Otherwise you should use ICollection if you can; if it’s still not enough then IList.