What is IReadOnlyCollection<T> in .NET?

✅ What is IReadOnlyCollection<T> in .NET?

IReadOnlyCollection<T> is an interface in the .NET Framework that represents a read-only, enumerable collection of elements of type T.

It is defined in the System.Collections.Generic namespace and looks like this:

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> when:

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 can:

• See the items.
• Count the items.
• But not modify the list (Add, Remove, etc.).

🔁 Comparison to Other Interfaces

🧠 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>.