C#（十）：对象的类型转换

向上转型(隐式类型转换) - 子类转换成父类

• 向上转型一定成功

Dog dog = new Dog();
Animal animal = dog;
// => Animal animalDog = new Dog();
// => sbyte b = 1; int a = b; => int ab = 1;

class Animal { }
class Dog : Animal { }
class Hashiqi : Dog { }

• 示例：动物园动物录入

class MainClass
{
    public static void Main(string[] args)
    {
        Animal.RecordAnimal(new Bird()); // 录入动物
        // => 详单于
        // Animal animal = new Bird(); // 向上转型
        // Animal.RecordAnimal(animal);
    }
}

class Animal {
    public static void RecordAnimal(Animal animal) { }
}
class Bird : Animal { }
class Monkey : Animal { }
class Tiger : Animal { }

• 向上转型后的对象将不再能够使用子类中特有的字段属性和方法

class MainClass
{
    public static void Main(string[] args)
    {
        Hashiqi hashiqi = new Hashiqi();
        hashiqi.type = ""; // ok
        hashiqi.sex = ' '; // ok
        hashiqi.name = ""; // ok
        hashiqi.age = 1; // ok


        Dog dog = new Dog();
        //dog.type = ""; // Error “Dog”未包含“type”的定义，并且找不到可接受第一个“Dog”类型参数的可访问扩展方法“type”
        dog.sex = ' '; // ok
        dog.name = ""; // ok
        dog.age = 1; // ok

        Animal animal = new Animal();
        //animal.type = ""; // Error “Animal”未包含“type”的定义，并且找不到可接受第一个“Animal”类型参数的可访问扩展方法“type”
        //animal.sex = ' '; // Error “Animal”未包含“sex”的定义，并且找不到可接受第一个“Animal”类型参数的可访问扩展方法“sex”
        animal.name = ""; // ok
        animal.age = 1; // ok


        Animal d = new Hashiqi();
        //d.type = ""; // Error “Animal”未包含“type”的定义，并且找不到可接受第一个“Animal”类型参数的可访问扩展方法“type”
        //d.sex = ' '; // Error “Animal”未包含“sex”的定义，并且找不到可接受第一个“Animal”类型参数的可访问扩展方法“sex”
        d.name = ""; // ok
        d.age = 1; // ok

        Dog dd = new Hashiqi();
        //dd.type = ""; // Error “Dog”未包含“type”的定义，并且找不到可接受第一个“Dog”类型参数的可访问扩展方法“type”
        dd.sex = ' '; // ok
        dd.name = ""; // ok
        dd.age = 1; // ok
    }
}

class Animal {
    public string name;
    public int age;
}
class Dog : Animal {
    public char sex;
}
class Hashiqi : Dog {
    public string type;
}

向下转型(需要强制类型转换) - 父类转换成子类

• 如果向下转型失败，那么得到​​null​​

// Dog dog = (Dog)new Animal(); // 不推荐
Dog dog = new Animal() as Dog;
Console.WriteLine(dog == null); // True

class Animal { }
class Dog : Animal { }
class Hashiqi : Dog { }

• 转型成功

class MainClass
{
    public static void Main(string[] args)
    {
        Animal animal = new Dog(); // 向上转型
        Dog dog = animal as Dog; // 向下转型

        Console.WriteLine(dog); // Dog
        Console.WriteLine(dog == null); // False
    }
}

class Animal { }
class Dog : Animal { }
class Hashiqi : Dog { }

• 向下转型错误示例

class MainClass
{
    public static void Main(string[] args)
    {
        Hashiqi hashiqi = new Animal() as Hashiqi; // 向下转型
        Console.WriteLine(hashiqi == null); // True
        Console.WriteLine(hashiqi.type == null); // Error System.NullReferenceException
    }
}

class Animal {
    public string name;
    public int age;
}
class Dog : Animal {
    public char sex;
}
class Hashiqi : Dog {
    public string type;
}