An empty set implementation

EmptySet.cs

#region Copyright (c) 2021 Atif Aziz. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

sealed partial class EmptySet<T> : ISet<T>
{
    public static readonly EmptySet<T> Instance = new EmptySet<T>();

    EmptySet() { }

    public int Count => 0;
    public bool IsReadOnly => true;
    public bool Contains(T item) => false;
    void ICollection<T>.CopyTo(T[] array, int arrayIndex) { /* nothing to copy */ }
    public IEnumerator<T> GetEnumerator() { yield break; }
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    public bool IsProperSubsetOf(IEnumerable<T> other) => true;
    public bool IsProperSupersetOf(IEnumerable<T> other) => false;
    public bool IsSubsetOf(IEnumerable<T> other) => true;
    public bool IsSupersetOf(IEnumerable<T> other) => false;
    public bool Overlaps(IEnumerable<T> other) => false;

    public bool SetEquals(IEnumerable<T> other)
    {
        switch (other)
        {
            case null: throw new ArgumentNullException(nameof(other));
            case ICollection<T> collection: return collection.Count == 0;
            case IReadOnlyCollection<T> collection: return collection.Count == 0;
            case { } sequence:
            {
                using var e = sequence.GetEnumerator();
                return !e.MoveNext();
            }
        }
    }

    void ICollection<T>.Add(T item) => throw new NotSupportedException();
    bool ICollection<T>.Remove(T item) => throw new NotSupportedException();
    void ISet<T>.ExceptWith(IEnumerable<T> other) => throw new NotSupportedException();
    void ISet<T>.IntersectWith(IEnumerable<T> other) => throw new NotSupportedException();
    void ISet<T>.SymmetricExceptWith(IEnumerable<T> other) => throw new NotSupportedException();
    void ISet<T>.UnionWith(IEnumerable<T> other) => throw new NotSupportedException();
    bool ISet<T>.Add(T item) => throw new NotSupportedException();
    void ICollection<T>.Clear() => throw new NotSupportedException();
}