C#: What is the difference between IEnumerable and ICollection?

IEnumerable — read-only iteration. ICollection — read/write with count.

IEnumerable:

• Forward iteration only.
• No count.
• Deferred.

ICollection:

• Add, Remove, Contains.
• Count property.
• Materialized.

1// IEnumerable
2IEnumerable<int> query = list.Where(x => x > 5);
3
4// ICollection
5ICollection<int> collection = new List<int> { 1, 2, 3 };
6collection.Add(4);
7collection.Remove(1);
8bool hasTwo = collection.Contains(2);

Key: IEnumerable for iteration, ICollection for manipulation.