协变(List泛型作为方法参数时的父类子类问题)
共 5363字，需浏览 11分钟
·
2024-05-30 08:00
有段时间没搞.net的项目了（没办法，谁让国内JAVA流行是事实）。最近又回归.net（哪里需要哪里搬~）。
接收到需求后，一顿输出，结果...咦?编译失败???
错误信息：
CS1503:参数1:无法从"System.Collections.Generic.List<Student>" 转换为"Person" 额...仔细回想下(好久不搞.net，.net的理论知识都已经落灰了)。
下面是完整的测试代码：
public class Person{public string Name { get; set; }public int Age { get; set; }}public class Student :Person{public string School { get; set; }public decimal Score { get; set; }}public abstract class Point{public Person person { get; set; }public List<Person> persons { get; set; }/// <summary>/// 单个/// </summary>public abstract void Info();/// <summary>/// 批量/// </summary>public abstract void List();}public class PointStudent : Point{public PointStudent(Person data){this.person = data;}public PointStudent(List<Person> data){this.persons = data;}public override void Info(){Console.WriteLine($"{((Student)person).Name}就读于{((Student)person).School}");}public override void List(){this.persons.ForEach(p => { this.person = p; Info(); });}}static void Main(string[] args){Student student = new Student() { Name="张三",Age = 18,School ="东师"};Point point = new PointStudent(student);point.Info();List<Student> students = new List<Student>() { student };Point pointList = new PointStudent(students);pointList.List();}
那么问题来了..
为什么使用子类的对象作为参数传入时没有错误？
答：
后期绑定（Polymorphism）：C# 支持后期绑定，这意味着在运行时确定方法的实际执行版本，而不是在编译时确定。
继承和多态性：子类继承了基类的所有特性，同时可以拥有自己的特性和行为。当你将子类对象传递给期望基类类型的方法时，方法将根据子类对象的实际类型执行相应的操作。
为什么使用List时报错？
答：
对于List<Person>
// Covariance.IEnumerable<string> strings = new List<string>();// An object that is instantiated with a more derived type argument// is assigned to an object instantiated with a less derived type argument.// Assignment compatibility is preserved.IEnumerable<object> objects = strings;
[TypeDependency("System.SZArrayHelper")][__DynamicallyInvokable]public interface IEnumerable<out T> : IEnumerable{[__DynamicallyInvokable]new IEnumerator<T> GetEnumerator();}
public class Person{public string Name { get; set; }public int Age { get; set; }}public class Student :Person{public string School { get; set; }public decimal Score { get; set; }}public abstract class Point{public Person person { get; set; }public IEnumerable<Person> persons { get; set; }/// <summary>/// 单个/// </summary>public abstract void Info();/// <summary>/// 批量/// </summary>public abstract void List();}public class PointStudent : Point{public PointStudent(Person data){this.person = data;}public PointStudent(IEnumerable<Person> data){this.persons = data;}public override void Info(){Console.WriteLine($"{((Student)person).Name}就读于{((Student)person).School}");}public override void List(){this.persons.ForEach(p => { this.person = p; Info(); });}}static void Main(string[] args){Student student = new Student() { Name="张三",Age = 18,School ="东师"};Point point = new PointStudent(student);point.Info();List<Student> students = new List<Student>() { student };Point pointList = new PointStudent(students);pointList.List();}