DelegateEqualityComparer.cs

    public class DelegateEqualityComparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, T, bool> equalityPredicate;
        private readonly Func<T, int> hashCodePredicate;

        public DelegateEqualityComparer(Func<T, T, bool> equalityPredicate)
            : this (equalityPredicate, null)
        {
        }

        public DelegateEqualityComparer(Func<T, T, bool> equalityPredicate, Func<T, int> hashCodePredicate)
        {
            this.equalityPredicate = equalityPredicate;
            this.hashCodePredicate = hashCodePredicate;
        }

        public bool Equals(T? x, T? y)
        {
            if (x is null && y is null)
                return true;
            if (x is null || y is null)
                return false;
            return equalityPredicate.Invoke(x, y);
        }

        public int GetHashCode([DisallowNull] T obj)
        {
            return hashCodePredicate?.Invoke(obj) ?? obj.GetHashCode();
        }
    }