public class Animal
{
    public int _age;
    public Animal(int age)
    {
      this._age = age;
    }
}
public class Dog : Animal
{
    public float _weight;
    public Dog(float weight, int age) : base(age)
    {
      _weight = weight;
    }
}

static void Main(string[] args)
{
  Dog dog = new Dog(2.5f,12);
  Animal animal = dog;
  Console.WriteLine(animal._age);
}

static void Main(string[] args)
{
  Animal animal = new Animal(12);
  Dog dog = (Dog)animal;
  Dog dog = animal as Dog;
  if (dog != null)
  {
 Console.WriteLine(dog._age);
  }
  else
  {
 Console.WriteLine("转换失败");
  }
}

static void Main(string[] args)
{
  Animal animal = new Animal(12);
  Dog dog = animal as Dog;
  if (dog != null)
  {
 Console.WriteLine(dog._age);
  }
  else
  {
 Console.WriteLine("转换失败");
  }
}

static void Main(string[] args)
{
  Animal animal = new Animal(12);
  Dog dog = null;
  if (animal is Dog)
  {
 dog = (Dog)animal;
  }
  if (dog == null)
  {
 Console.WriteLine("转换失败");
  }
  else
  {
 Console.WriteLine("转换成功");
  }
}

public class Benz
{
    public int Mile { get; set; }
    public Benz(int mile)
    {
      Mile = mile;
    }
    public static implicit operator Car(Benz benz)
    {
      return new Car(){Mile = benz.Mile};
    }
}
public class Car
{
public int Mile { get; set; }
}

static void Main(string[] args)
{
  Benz benz = new Benz(1000);
  Car car = benz;
  Console.WriteLine(car.Mile);
  Console.ReadKey();
}