添加Lesson20

C#核心/Lesson20_多态-密封方法/Lesson20_多态-密封方法.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <RootNamespace>Lesson20_多态_密封方法</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

C#核心/Lesson20_多态-密封方法/Program.cs

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+namespace Lesson20_多态_密封方法
+{
+    internal class Program
+    {
+        #region 知识回顾
+        //用sealed来防止类被继续继承
+        #endregion
+
+        #region 知识点一 密封方法的基本概念
+        //用密封关键字sealed修饰的重写函数
+        //作用：让虚方法或者抽象方法之后不能被重写
+        //特点：和override一起出现
+        #endregion
+
+
+        #region 知识点二 实例
+        abstract class Animal
+        {
+            public string name;
+            public abstract void Eat();
+            public virtual void Speak() {
+                Console.WriteLine("123");
+            }
+        }
+        class Person : Animal
+        {
+            public override void Eat()
+            {
+                Console.WriteLine("123");
+            }
+            public override void Speak()
+            {
+                Console.WriteLine("spaek");
+            }
+        }
+        class WhitePerson : Person
+        {
+            public sealed override void Speak()
+            {
+                base.Speak();
+            }
+            public sealed override void Eat()
+            {
+                base.Eat();
+            }
+        }
+        #endregion
+        static void Main(string[] args)
+        {
+            Console.WriteLine("Hello, World!");
+        }
+    }
+}