C# 子类与父类互转注意项

昨晚在处理父类与子类相互转换时，想把父类转换子类对象，发现编译不通过，类定义如下:

 public interface IPeople

    {

        int Age {

            get;

            set;

        }

        string Name {

            get;

            set;

        }

    }

   public class People : IPeople

    {

        public int Age { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

        public string Name { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

    }

public class Student : People

    {

    }

测试代码:

 IPeople p = new People();

  Student stu = (Student)p;

这里， People 继承 IPeople , Student 继承 People , 即 Student 是 People 子类，先创建父类对象，原后强转子类，运行报错：

如上，换个方式， Student , People 均继承 IPeople , 试试看:

 public class People : IPeople

    {

        public int Age { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

        public string Name { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

    }

  public class Student : IPeople

    {

        public int Age { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

        public string Name { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }

    }

一样报错:

这里 Student 与 People 均继承 IPeople , 仍报错， Student 与 People 不能互相转换，但二者可以转成 IPeople ，做为通用方法传参使用。

如：

  static void Print(IPeople p) {

            Console.WriteLine($"age:{p.Age}");

            Console.WriteLine($"name:{p.Name}");

      }

另一种合法转换，如子类转父类是充许的，在如上第一示例基础上，运行如下测试代码，一切正常:

  IPeople p = new Student();

  Student stu = (Student)p;

这里可以推测在内存中创建（New Student()）对象，本质上就是 Student ，同理创建（New People()）对象，本质上就是 People , 之所以子类能够转换父类，只是逻辑层转换(内存结构不变)，因为子类继承父类所有功能属性。逻辑层面，子类转成父类后，能够像调用父类方法一样调用子类。

由于父类无法转换子类，因此只能自个写一些转换逻辑，比如在子类构造中传入父类对象，显示将父类属性copy 至子类，考虑到 copy 繁琐，可借助反射属性方式自动同步。

  public class People : IPeople

    {

        public int Age {

            get;

            set;

        }

        public string Name {

            get;

            set;

        }

    }

 public class Student : People

    {

        public Student() { }

        public Student(People peo) {

            SynchronizationProperties(peo, this);

        }

        void SynchronizationProperties(object src , object des) {

            Type srcType = src.GetType();

            object val;

            foreach (var item in srcType.GetProperties())

            {

                val = item.GetValue(src);

                item.SetValue(des, val );

            }

        }

    }

调用代码:

 //创建父类对象

            People p = new People() { Age = 18, Name = "张三" };

            //将父类对象传入子类，Copy 公共属性

            Student stu = new Student(p);

            Console.WriteLine($"Name:{stu.Name} , Age:{stu.Age}");

输出结果:

巴特西

C# 子类与父类互转注意项

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