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

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:

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.
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.
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

| Interface | Can Modify? | Has Count? | Enumerator? | Intent | | ------------------------ | ----------- | ---------- | ----------- | --------------------------- | |

IEnumerable<T>

| ❌ | ❌ | ✅ | Enumeration only | |

IReadOnlyCollection<T>

| ❌ | ✅ | ✅ | Read-only access with count | |

ICollection<T>

| ✅ | ✅ | ✅ | Modifiable collection | |

List<T>

T[]

| ✅ | ✅ | ✅ | Concrete mutable collection |

🧠 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>
```
.

What is IReadOnlyCollection<T> in .NET?