IEnumerable vs ICollection vs IList?

IEnumerable vs ICollection vs IList

What interviewers test

• Execution pipeline understanding
• Abstraction knowledge

Hierarchy

IEnumerable

└── ICollection

└── IList

IEnumerable<T>

• Read-only iteration
• Lazy execution

IEnumerable<int> numbers = GetNumbers();
foreach (var n in numbers) { }

✅ Best for streaming, read-only, deferred execution

ICollection<T>

• Adds count + add/remove

ICollection<int> list = new List<int>();
list.Add(1);

✅ When modifying collection without index access

IList<T>

• Index-based access

IList<int> list = new List<int>();
int first = list[0];

✅ When order & index matter

Interview rule of thumb

“Expose the least powerful interface required.”