new и override при вызове через базовый тип

class Base
{
    public string Hello() => "Base";
    public virtual string Bye() => "Base";
}
class Derived : Base
{
    public new string Hello() => "Derived";
    public override string Bye() => "Derived";
}

Base b = new Derived();
Console.WriteLine($"{b.Hello()} {b.Bye()}");