增加C#4。0

data/1..NET初阶/3.C#特性/2.C#4.0特性/Contravariance.json

+{
+  "type": "code_options",
+  "author": "huanhuilong",
+  "source": "Contravariance.md",
+  "notebook_enable": false,
+  "exercise_id": "12348524087540ceb975cd9f2d00108f"
+}
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/Contravariance.md

+# 逆变(Contravariance)
+
+假设有一个父类和一个子类：
+
+```csharp
+// 父类和子类
+class Article{
+    public string ToString(){
+        return "article";
+    }
+}
+
+class CodeArticle: Article{
+    public string ToString(){
+        return "article with code";
+    }
+}
+```
+
+那么，通常针对父类的方法是可以传入子类实例的，4.0增加了`逆变(Contravariance)`，也就是父类的泛型实现可以转成子类的泛型接口。
+
+
+现在有如下代码：
+
+```csharp
+// 逆变
+public interface ISend<in T> {    
+    void Send(T t);
+}
+
+public class ArticleSender<T> : ISend<T>{
+    public void Send(T t){
+        Console.WriteLine("Send:{0}", t.ToString());
+    }
+}
+```
+
+以下代码正确的是？
+
+## 答案
+
+```csharp
+ISend<CodeArticle> sender = new ArticleSender<Article>();
+sender.Send(new CodeArticle());
+```
+
+## 选项
+
+### A
+
+```csharp
+ISend<Article> sender = new ArticleSender<CodeArticle>();
+sender.Send(new Article());
+```
+
+### B
+
+```csharp
+ISend<Article> sender = new ArticleSender<CodeArticle>();
+sender.Send(new CodeArticle());
+```
+
+### C
+
+```csharp
+ISend<CodeArticle> sender = new ArticleSender<Article>();
+sender.Send(new Article());
+```
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/Covariance.json

+{
+  "type": "code_options",
+  "author": "huanhuilong",
+  "source": "Covariance.md",
+  "notebook_enable": false,
+  "exercise_id": "0b9afd8bc5ac4139af83b90f7211d41a"
+}
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/Covariance.md

+# 协变(Covariance)
+
+假设有一个父类和一个子类：
+
+```csharp
+// 父类和子类
+class Article{
+    public string ToString(){
+        return "article";
+    }
+}
+
+class CodeArticle: Article{
+    public string ToString(){
+        return "article with code";
+    }
+}
+```
+
+那么，通常可以把子类当作父类来使用，但是如果包了一层泛型在4.0之前是不可以的：
+
+```csharp
+IEnumerable<object> objs = new List<string> { "Keep it simple, stupid" };
+```
+
+4.0增加了`协变(Covariance)`特性后则可以支持上述代码，也就是子类的泛型可以转成父类的接口。
+
+
+现在有如下代码：
+
+```csharp
+// 协变
+public interface ISearch<out T> {    
+    T Search();
+}
+
+public class CodeArticleSearch<T>: ISearch<T> where T:new(){
+    public T Search(){
+        return new T();
+    }
+}
+```
+
+以下代码正确的是？
+
+## 答案
+
+```csharp
+ISearch<Article> search = new CodeArticleSearch<CodeArticle>();
+Article article = search.Search();
+Console.WriteLine(article);
+```
+
+## 选项
+
+### A
+
+```csharp
+ISearch<CodeArticle> search = new CodeArticleSearch<Article>();
+Article article = search.Search();
+Console.WriteLine(article);
+```
+
+### B
+
+```csharp
+ISearch<Article> search = new CodeArticleSearch<CodeArticle>();
+CodeArticle article = search.Search();
+Console.WriteLine(article);
+```
+
+### C
+
+```csharp
+ISearch<CodeArticle> search = new CodeArticleSearch<>(Article);
+CodeArticle article = search.Search();
+Console.WriteLine(article);
+```
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/NamedParam.json

+{
+  "type": "code_options",
+  "author": "huanhuilong",
+  "source": "NamedParam.md",
+  "notebook_enable": false,
+  "exercise_id": "eb2c4ef49ddc42feb74258f81f3c3504"
+}
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/NamedParam.md

+# 命名参数
+
+4.0引入了命名参数，命名参数让我们可以在调用方法时指定参数名字来给参数赋，还可以调整参数顺序
+
+```csharp
+static class Test{
+    public static List<string> Find(string key, int number, bool sort){
+        Console.WriteLine("key:{0}, number:{1}, sort:{2}", key, number, sort);
+        return new List<string>(){};
+    }
+}
+```
+
+以下对函数调用错误的是？
+
+
+## 答案
+
+```csharp
+Test.Find(key:"hello", number:5);
+```
+
+## 选项
+
+### A
+
+```csharp
+Test.Find(key:"hello", number:5, sort:false);
+```
+
+### B
+
+```csharp
+Test.Find(key:"hello", sort:false, number:5);
+```
+
+### C
+
+```csharp
+Test.Find(sort:false, number:5, key:"hello");
+```
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/OptionParam.json

+{
+  "type": "code_options",
+  "author": "huanhuilong",
+  "source": "OptionParam.md",
+  "notebook_enable": false,
+  "exercise_id": "0cccd130210a4af6aea30df02619a70d"
+}
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/OptionParam.md

+# 可选参数
+
+4.0引入了可选参数，如下的函数，示例代码如下
+
+```csharp
+static class Test{
+    public static List<string> Find(string key, int number=5, bool sort=false){
+        Console.WriteLine("key:{0}, number:{1}, sort:{2}", key, number, sort);
+        return new List<string>(){};
+    }
+}
+```
+
+以下对函数调用错误的是？
+
+
+## 答案
+
+```csharp
+Test.Find("hello", true, 10);
+```
+
+## 选项
+
+### A
+
+```csharp
+Test.Find("hello");
+```
+
+### B
+
+```csharp
+Test.Find("hello", 10);
+```
+
+### C
+
+```csharp
+Test.Find("hello", 10, true);
+```
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/config.json

 {
   "node_id": "csharp-d4650354f5f5498e9969fd98bbb40323",
   "keywords": [
-    "互操作特性",
     "协变和逆变",
-    "命名参数和可选参数",
+    "命名参数",
+    "可选参数",
     "动态查找"
   ],
   "children": [],
-  "export": [],
+  "export": [
+    "OptionParam.json",
+    "NamedParam.json",
+    "Covariance.json",
+    "Contravariance.json"
+  ],
   "keywords_must": [],
   "keywords_forbid": []
 }
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/sample/Program.cs

+// See https://aka.ms/new-console-template for more information
+Console.WriteLine("Hello, World!");
+
+// // // 协变代码
+// ISearch<Article> search = new CodeArticleSearch<CodeArticle>();
+// Article article = search.Search();
+// Console.WriteLine(article);
+
+// // 逆变代码
+// ISend<CodeArticle> sender = new ArticleSender<Article>();
+// sender.Send(new CodeArticle());
+
+
+// Test.Find("hello");
+// Test.Find("hello", 10);
+// Test.Find("hello", 10, true);
+// Test.Find("hello", true, 10);
+
+
+// Test.Find(key:"hello", number:5);
+Test.Find(key:"hello", sort:false, number:5);
+Test.Find(sort:false, number:5, key:"hello");
+
+
+static class Test{
+    public static List<string> Find(string key, int number, bool sort){
+        Console.WriteLine("key:{0}, number:{1}, sort:{2}", key, number, sort);
+        return new List<string>(){};
+    }
+}
+
+
+// 父类和子类
+class Article{
+    public string ToString(){
+        return "article";
+    }
+}
+
+class CodeArticle: Article{
+    public string ToString(){
+        return "article with code";
+    }
+}
+
+
+// 协变
+public interface ISearch<out T> {    
+    T Search();
+}
+
+public class CodeArticleSearch<T>: ISearch<T> where T:new(){
+    public T Search(){
+        return new T();
+    }
+}
+
+// 逆变
+public interface ISend<in T> {    
+    void Send(T t);
+}
+
+public class ArticleSender<T> : ISend<T>{
+    public void Send(T t){
+        Console.WriteLine("Send:{0}", t.ToString());
+    }
+}
\ No newline at end of file

data/1..NET初阶/3.C#特性/2.C#4.0特性/sample/sample.csproj

+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

data/tree.json

@@ -210,9 +210,9 @@
                     "C#4.0特性": {
                       "node_id": "csharp-d4650354f5f5498e9969fd98bbb40323",
                       "keywords": [
-                        "互操作特性",
                         "协变和逆变",
-                        "命名参数和可选参数",
+                        "命名参数",
+                        "可选参数",
                         "动态查找"
                       ],
                       "children": [],