添加Lesson21

C#核心/Lesson21_面向对象相关-命名空间练习/Lesson21_面向对象相关-命名空间练习.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <RootNamespace>Lesson21_面向对象相关_命名空间练习</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

C#核心/Lesson21_面向对象相关-命名空间练习/Program.cs

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+namespace Lesson21_面向对象相关_命名空间练习
+{
+    namespace UI
+    {
+        class Image
+        {
+        }
+    }
+    namespace Graph
+    {
+        class Image
+        {
+        }
+    }
+    internal class Program
+    {
+        static void Main(string[] args)
+        {
+            Console.WriteLine("Hello, World!");
+            UI.Image img1 = new UI.Image();
+            Graph.Image img2 = new Graph.Image();
+        }
+    }
+}