DotNext by Roman Sakno

<PackageReference Include="DotNext" Version="0.3.0" />

.NET API 112,128 bytes

SingletonList<T>

sealed class SingletonList<T> : Tuple<T>, IReadOnlyList<T>, IEnumerable<T>, IEnumerable, IReadOnlyCollection<T>

public SingletonList(T item)

public Enumerator<T> GetEnumerator()