Umbraco-CMS/src/Umbraco.Core/Composing/LazyReadOnlyCollection.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace Umbraco.Cms.Core.Composing
{
    public sealed class LazyReadOnlyCollection<T> : IReadOnlyCollection<T>
    {
        private readonly Lazy<IEnumerable<T>> _lazyCollection;
        private int? _count;

        public LazyReadOnlyCollection(Lazy<IEnumerable<T>> lazyCollection) => _lazyCollection = lazyCollection;

        public LazyReadOnlyCollection(Func<IEnumerable<T>> lazyCollection) => _lazyCollection = new Lazy<IEnumerable<T>>(lazyCollection);

        public IEnumerable<T> Value => EnsureCollection();

        private IEnumerable<T> EnsureCollection()
        {
            if (_lazyCollection == null)
            {
                _count = 0;
                return Enumerable.Empty<T>();
            }

            IEnumerable<T> val = _lazyCollection.Value;
            if (_count == null)
            {
                _count = val.Count();
            }
            return val;
        }

        public int Count
        {
            get
            {
                EnsureCollection();
                return _count.Value;
            }
        }

        public IEnumerator<T> GetEnumerator() => Value.GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }
}