对象引用可以隐式向上转换为基类引用显式地向下转换为子类引用
Plant是PositivePlant和NegativePlant的基类
PositivePlant positivePlant = new PositivePlant() { Name = "阳性植物", MinimumSurvivalTemperature = 10 };
//子转基：隐式
Plant plant = positivePlant;
//Plant plant = (Plant)positivePlant;//正确的写法
//基转子：显式
PositivePlant convertFromPlant = (PositivePlant)plant;
//PositivePlant convertFromPlant = plant;//错误的写法
Console.WriteLine($"positivePlant == plant:{positivePlant == plant}");//true
Console.WriteLine($"positivePlant == convertFromPlant:{positivePlant == convertFromPlant}");//true
//as运算符
Plant plant2 = positivePlant as Plant;
PositivePlant convertFromPlant2 = plant2 as PositivePlant;
Console.WriteLine($"positivePlant == plant2:{positivePlant == plant2}");//true
Console.WriteLine($"positivePlant == convertFromPlant2:{positivePlant == convertFromPlant2}");//true