public class A
{
    public A()
    {
        Console.WriteLine("мир!");
    }
}

public class B
{
    public B()
    {
        Console.WriteLine("Привет ");
    }
}

public class A
{
    public A()
    {
        Console.Write("Hello ");
    }
}

public class B: A
{
    public B() : base()
    {
        Console.Write("world!");
    }
}

class Program
{
    static void Main(string[] args)
    {
        var b = new B();
    }
}