Connected.Framework/Connected.Expressions/Collections/CollectionExtensions.cs

using System.Collections.ObjectModel;

namespace Connected.Expressions.Collections;

internal static class Extensions
{
    public static ReadOnlyCollection<T> ToReadOnly<T>(this IEnumerable<T> sequence)
    {
        if (sequence is not ReadOnlyCollection<T> collection)
        {
            if (sequence is null)
                collection = EmptyReadOnlyCollection<T>.Empty;
            else
                collection = new List<T>(sequence).AsReadOnly();
        }

        return collection;
    }

    private class EmptyReadOnlyCollection<T>
    {
        public static readonly ReadOnlyCollection<T> Empty = new List<T>().AsReadOnly();
    }
}