Return an ICollection<T> as IReadOnlyCollection<T>?

Question

I have a field of type ICollection<T>. Is there a simple way to make it read only?

Note, the ReadOnlyCollection<T> class takes an IList<T>, I'm looking for a way to invert an ICollection<T> and ensure user cannot alter the collection.

Using AsEnumerable ex. method, won't guarantee that, since it could be casted back to its original type, which could also be List.

Answer 1

I've always thought it a little odd that ReadOnlyCollection<T> is really a read-only list.

That said, ICollection<T> is such a simple interface, it's trivial to implement a true ReadOnlyCollection<T>:

public class TrueReadOnlyCollection<T> : ICollection<T>, IReadOnlyCollection<T>
{
    private ICollection<T> _original;

    public TrueReadOnlyCollection(ICollection<T> original)
    {
        _original = original;
    }

    public void Add(T item)
    {
        throw new NotSupportedException();
    }

    public void Clear()
    {
        throw new NotSupportedException();
    }

    public bool Contains(T item)
    {
        return _original.Contains(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        _original.CopyTo(array, arrayIndex);
    }

    public int Count
    {
        get { return _original.Count; }
    }

    public bool IsReadOnly
    {
        get { return true; }
    }

    public bool Remove(T item)
    {
        throw new NotSupportedException();
    }

    public IEnumerator<T> GetEnumerator()
    {
        foreach (T t in _original)
        {
            yield return t;
        }
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Note that with .NET 4.5 there are also actual read-only interfaces to support the newish interface-variance features, so if you're doing the above it might be nice to also implement IReadOnlyCollection<T>