<PackageReference Include="NETStandard.Library" Version="2.0.2" />

System.Collections.ObjectModel.ReadOnlyCollection<T>

namespace System.Collections.ObjectModel { public class ReadOnlyCollection<T> : ICollection<T>, IEnumerable<T>, IEnumerable, IList<T>, IReadOnlyCollection<T>, IReadOnlyList<T>, ICollection, IList { public int Count { get; } public T this[int index] { get; } protected IList<T> Items { get; } T IList<T>.this[int index] { get; set; } object IList.this[int index] { get; set; } public ReadOnlyCollection(IList<T> list); public bool Contains(T value); public void CopyTo(T[] array, int index); public IEnumerator<T> GetEnumerator(); public int IndexOf(T value); } }