What is the difference between IEnumerable<T> and ICollection<T> in C#?

Short answer: Feature IEnumerable <T> ICollection<T> Read-only Yes No Modification Not supported Supports Add, Remove, Clear Count property No Yes (Count)

Example code

IEnumerable<string> names = new List<string> { "A", "B" }; // names.Add("C"); // Error ICollection<string> nameCollection = new List<string> { "A", "B" }; nameCollection.Add("C"); // Allowed Real-world scenario: Use IEnumerable<T> for read-only, query-focused tasks like LINQ. Use ICollection<T> when managing and modifying the collection.

Real-world example (ShopNest)

In ShopNest, an order page loads products with List<Product> so you can Add items to the cart and access them by index. Use IEnumerable<T> when you only need to loop (for example, printing invoice lines).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.