Collections in C#

Collections in C# are essential for managing groups of related data. The .NET Framework provides a hierarchy of collection interfaces and classes. Among these, three key interfaces often come up in day-to-day development

IEnumerable

• Read-only, forward-only access – Ideal for looping through data using foreach. You can’t modify the collection.
• Deferred Execution (Lazy Evaluation) – LINQ queries return IEnumerable, and the actual processing doesn't happen until you iterate over the collection.
• Minimal Overhead – Great for when you only need to read data.

Use Case:
Filtering or transforming data from a database or in-memory source without changing it.

ICollection

• Supports Add/Remove/Clear – Unlike IEnumerable, it allows modifying the collection contents.
• Exposes Count property – Useful to check collection size efficiently.
• Abstracts concrete collections – Useful when writing APIs that shouldn’t care about specific collection types (e.g., List, HashSet, etc.).

Use Case:
Passing modifiable collections between services or layers without exposing underlying data structure.

IList

• Index-Based Access – Allows reading and writing via list[index].
• Insert and Remove by Index – Easily insert or delete items at specific positions.
• Ideal for Ordered Data – Useful for collections where order and position matter.

Use Case:
Manage an ordered list where you frequently insert, update, or remove items by position (e.g., UI elements, to-do list).

Conclusion

In C#, choosing the right collection interface—IEnumerable, ICollection, or IList—depends on the specific needs of your application.

Each interface builds upon the previous one, adding more capabilities:

IEnumerable → ICollection → IList

By understanding their differences, you can write cleaner, more flexible, and maintainable code tailored to your application's needs.