ReadOnlyCollectionBase.cs

using System.Runtime.InteropServices;

namespace System.Collections
{
    [Serializable]
    [ComVisible(true)]
    public abstract class ReadOnlyCollectionBase : ICollection, IEnumerable
    {
        private ArrayList list;

        protected ArrayList InnerList
        {
            get
            {
                if (list == null)
                {
                    list = new ArrayList();
                }
                return list;
            }
        }

        public virtual int Count => InnerList.Count;

        bool ICollection.IsSynchronized
        {
            get
            {
                return InnerList.IsSynchronized;
            }
        }

        object ICollection.SyncRoot
        {
            get
            {
                return InnerList.SyncRoot;
            }
        }

        void ICollection.CopyTo(Array array, int index)
        {
            InnerList.CopyTo(array, index);
        }

        public virtual IEnumerator GetEnumerator()
        {
            return InnerList.GetEnumerator();
        }
    }
}