public static void Show()
{
{
Bird bird1 = new Bird();
Bird bird2 = new Sparrow(); // 子类实例化
Sparrow sparrow1 = new Sparrow();
// Sparrow sparrow2 = new Bird(); // 子类变量不能用父类实例化
}
{
List <Bird> birdList1 = new List <Bird>();
// List<Bird> birdList2 = new List<Sparrow>(); // error: 语法错误：List<Bird>是一个类，List<Sparrow>也是一个类，两者没有父子关系，List<> 之间没有关系。
List <Bird> birdList3 = new List <Sparrow>().Select(c => (Bird)c).ToList();
}
{
// 协变：就是可以让右边用上子类
// out 修饰后，T 只能作为返回值，不能当参数
IEnumerable <Bird> birdList1 = new List <Bird>();
IEnumerable <Bird> birdList2 = new List <Sparrow>();
Func <Bird> func = new Func <Sparrow>(() => null);
ICustomerListOut <Bird> customerList1 = new CustomerListOut <Bird>();
ICustomerListOut <Bird> customerList2 = new CustomerListOut <Sparrow>();
customerList2.Get();
}
{
// 逆变：就是可以让右边用上父类
// in 修饰后，T 只能作为参数，不能当返回值
ICustomerListIn <Sparrow> customerList1 = new CustomerListIn <Bird>();
ICustomerListIn <Sparrow> customerList2 = new CustomerListIn <Sparrow>();
customerList1.Show(new Sparrow());
ICustomerListIn <Bird> birdList1 = new CustomerListIn <Bird>();
birdList1.Show(new Sparrow());
birdList1.Show(new Bird());
Action <Sparrow> action = new Action <Bird>((Bird i) => { });
}
{
// 协变与逆变的组合
IMyList <Sparrow, Bird> myList1 = new MyList <Sparrow, Bird>();
IMyList <Sparrow, Bird> myList2 = new MyList <Sparrow, Sparrow>(); // 协变
IMyList <Sparrow, Bird> myList3 = new MyList <Bird, Bird>(); // 逆变
IMyList <Sparrow, Bird> myList4 = new MyList <Bird, Sparrow>(); // 协变 + 逆变
}
}
public static void Show()
{
{
Bird b1 = new Bird();
Bird b2 = new Sparrow();
Sparrow b3 = new Sparrow();
// Sparrow b4 = new Bird();
}
{
List <Bird> birds = new List <Bird>();
// List<Bird>birds1 = new List<Sparrow>();
List <Bird> birds2 = new List <Sparrow>()
.Select(c => ((Bird)c)).ToList();
}
{
// 协变
IEnumerable <Bird> birds = new List <Bird>();
IEnumerable <Bird> birds1 = new List <Sparrow>();
Func <Bird> func = new Func <Sparrow>(() => null);
//这是能编译的
ICustomerListOut <Bird> customerList1 = new CustomerListOut <Bird>();
//这也是能编译的，在泛型中，子类指向父类，我们称为协变
ICustomerListOut <Bird> customerList2 = new CustomerListOut <Sparrow>();
}
{
// 逆变
ICustomerListIn <Sparrow> customerList2 = new CustomerListIn <Sparrow>();
ICustomerListIn <Sparrow> customerList1 = new CustomerListIn <Bird>();//父类指向子类，我们称为逆变
ICustomerListIn <Bird> birdList1 = new CustomerListIn <Bird>();
birdList1.Show(new Sparrow());
birdList1.Show(new Bird());
Action <Sparrow> act = new Action <Bird>((Bird i) => { });
}
}