GenericCollection

GenericCollection.cs

        public class GenericCollection<T> : ICollection<T>
        {
            private readonly HashSet<T> _set; 

            public IEnumerator<T> GetEnumerator()
            {
                return _set.GetEnumerator();
            }

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

            public GenericCollection()
            {
                _set = new HashSet<T>();
            }

            public virtual void Add(T item)
            {
                _set.Add(item);
            }

            public virtual void Clear()
            {
                _set.Clear();
            }

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

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

            public virtual bool Remove(T item)
            {
               return _set.Remove(item);
            }

            public int Count => _set.Count;

            public bool IsReadOnly => false;
        }

        public class CustomerCollection : GenericCollection<Customer>
        {
            public override void Add(Customer item)
            {
                throw new NotImplementedException();
            }
        }