添加Lesson19

C#进阶/Lesson19_预处理器指令练习/Lesson19_预处理器指令练习.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

C#进阶/Lesson19_预处理器指令练习/Program.cs

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+
+
+#define Unity2017
+#define Unity2020
+
+
+namespace Lesson19_预处理器指令练习
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            Console.WriteLine("请输入第一个数字：");
+            int a = int.Parse(Console.ReadLine());
+
+            Console.WriteLine("请输入第二个数字：");
+            int b = int.Parse(Console.ReadLine());
+            Console.WriteLine(Calc(a, b));
+
+        }
+        static int Calc(int a, int b)
+        {
+#if Unity5
+            return a + b;
+#elif Unity2017
+            return a*b;
+#elif Unity2019
+            return a-b;
+#else
+            return 0;
+#endif
+        }
+    }
+}