C# Update a List from Another List

Answer 1

You could also use the Union method with an IEqualityComparer:

var newList = ListB.Union(ListA, new PersonEqualityComparer());

class PersonEqualityComparer : IEqualityComparer
{
    public bool Equals(Person person1, Person person2)
    {
        if (person1 == null && person2 == null)
            return true;
        else if ((person1 != null && person2 == null) ||
                (person1 == null && person2 != null))
            return false;

        return person1.Id.Equals(person2.Id);
    }

    public int GetHashCode(Person item)
    {
        return item.Id.GetHashCode();
    }
}