What is IReadOnlyCollection<T> in .NET?

✅ What is

IReadOnlyCollection<T>

in .NET?

IReadOnlyCollection<T>

T

It is defined in the

System.Collections.Generic

public interface IReadOnlyCollection<out T> : IEnumerable<T>
{
    int Count { get; }
}

So it's essentially:

• A read-only collection (you can't add/remove/modify items).
• You can enumerate it using

  foreach

  .
• You can get the number of elements via the

  Count

  property.

📌 When to Use

IReadOnlyCollection<T>

Use

IReadOnlyCollection<T>

1. You want to expose a collection as read-only:

   • Consumers of your code can read the elements but can't modify them.
   • It enforces immutability at the API boundary.

2. You want to convey intent clearly:

   • Using

     IEnumerable<T>

     only guarantees enumeration.
   • Using

     IReadOnlyCollection<T>

     adds the

     Count

     guarantee and communicates the intent that no mutation should happen.

3. Better than returning

   List<T>

   or

   T[]

   directly:
   • It avoids exposing unnecessary methods like

     Add

     ,

     Remove

     , etc.
   • Hides the underlying data structure.

✅ Example

public class Order
{
    private readonly List<string> _items = new List<string>();

    public void AddItem(string item)
    {
        _items.Add(item);
    }

    public IReadOnlyCollection<string> Items => _items;
}

This ensures that callers of

Order.Items

• See the items.
• Count the items.
• But not modify the list (

  Add

  ,

  Remove

  , etc.).

🔁 Comparison to Other Interfaces

IEnumerable<T>

IReadOnlyCollection<T>

ICollection<T>

List<T>

T[]

🧠 Tip

• IReadOnlyCollection<T>

  is especially useful in public APIs, DDD entities/aggregates, and library development, where encapsulation and immutability matter.
• Internally, you can still use a

  List<T>

  or

  HashSet<T>

  and expose it as

  IReadOnlyCollection<T>

  .