C#基础知识—父类和子类的关系

基础知识一：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public class ParentClass
    {
        public ParentClass()
        {

        }
        public string NamePropety { get; set; }

        public string GetName()
        {
            return "";
        }
    }

    public class ChildClass : ParentClass
    {
        public ChildClass()
        {

        }

        public int Age { get; set; }

        public int GetAge()
        {
            return 10;
        }
    }

    static class Program
    {
        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            //=>1、实例化父类
            ParentClass parent = new ParentClass();
            string _NamePropety = parent.NamePropety;
            string _name = parent.GetName();

            //1.1向上转型 子类转父类
            ParentClass parent1 = new ChildClass(); //或者ParentClass parent1 = new ChildClass() as ParentClass;
            string _NamePropety1 = parent1.NamePropety;
            string _name1 = parent1.GetName();


            //=>2、实例化子类
            ChildClass child = new ChildClass();
            string _NamePropety2 = child.NamePropety;
            string _name2 = child.GetName();
            int ageName2 = child.GetAge();
            int age2 = child.Age;


            //2.1向下转型 父类转换子类。
            ParentClass child3 = new ChildClass();
            ChildClass child4 = (ChildClass)child3;
            string _NamePropety3 = child4.NamePropety;
            string _name3 = child4.GetName();
            int ageName3 = child4.GetAge();
            int age3 = child4.Age;

            //=>3、不正确的父类转子类。

            //as方式转换。(as 转换失败时，程序不会抛异常，child1对象为NULL。)
            ChildClass child1 = new ParentClass() as ChildClass; //或者 ChildClass child1 = (ChildClass)new ParentClass();
            Console.WriteLine(child1.NamePropety);

            //强制转换。（程序会抛出异常。）
            ChildClass child1_1 = (ChildClass)new ParentClass();

            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }

      
    }
}

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