doc: update docs/cs.md (#609)
* 增加国内镜像 * add reference of syntax sugar for c# * optimizing page format for cs.md * optimizing page format for cs.md * add reference of linq for c#
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docs/cs.md
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docs/cs.md
@ -1065,6 +1065,267 @@ bool hasKey2 = hashTable.ContainsKey("key2");
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string valueOfKey2 = (string)hashTable["key2"];
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string valueOfKey2 = (string)hashTable["key2"];
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```
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```
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LINQ
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--------
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> C#语言中的LINQ（Language-Integrated Query）是一种强大的查询语言，它提供了一种统一的编程模型，使得数据查询变得更加直观和简洁。
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### FROM
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<!--rehype:wrap-class=col-span-3-->
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> 任何数据源，包括对象集合、数据库、XML等。
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### WHERE
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<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
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> 条件查询
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```cs
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// 示例数据源
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List<Student> students = new List<Student>
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{
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new Student { Name = "Alice", Age = 25, Grade = "A" },
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new Student { Name = "Bob", Age = 30, Grade = "B" },
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new Student { Name = "Barry", Age = 35, Grade = "C" },
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new Student { Name = "Charlie", Age = 22, Grade = "A" },
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new Student { Name = "David", Age = 21, Grade = "C" },
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new Student { Name = "Damon", Age = 28, Grade = "B" },
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new Student { Name = "Echo", Age = 18, Grade = "C" }
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};
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// 使用WHERE筛选出成绩为A的学生
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var result1 = students.Where(student => student.Grade = "A");
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// 使用WHERE筛选出年龄大于20的学生
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var result2 = students.Where(student => student.Age > 20);
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// 使用WHERE筛选出名字包含'D'的学生
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var result3 = students.Where(student => student.Name.Contains("D"));
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// 使用WHERE筛选出名字为'Bob'和'Echo'的学生
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List<string> nameList = new() { "Bob", "Echo" };
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var result4 = students.Where(student => nameList.Contains(student.Name));
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```
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### GROUPBY
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> 分组查询
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```cs
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// 示例数据源
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List<Student> students = new List<Student>
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{
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new Student { Name = "Alice", Age = 25, Grade = "A" },
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new Student { Name = "Bob", Age = 30, Grade = "B" },
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new Student { Name = "Barry", Age = 35, Grade = "C" },
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new Student { Name = "Charlie", Age = 22, Grade = "A" },
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new Student { Name = "David", Age = 21, Grade = "C" },
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new Student { Name = "Damon", Age = 28, Grade = "B" },
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new Student { Name = "Echo", Age = 18, Grade = "C" }
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};
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// 使用GROUP BY按成绩进行分组查询
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var groupedByGrade = students.GroupBy(student => student.Grade);
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```
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<!--rehype:className=wrap-text-->
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### SELECT
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<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
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> 结果查询
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```cs
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// 示例数据源
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List<Student> students = new List<Student>
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{
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new Student { Name = "Alice", Age = 25, Grade = "A" },
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new Student { Name = "Bob", Age = 30, Grade = "B" },
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new Student { Name = "Barry", Age = 35, Grade = "C" },
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new Student { Name = "Charlie", Age = 22, Grade = "A" },
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new Student { Name = "David", Age = 21, Grade = "C" },
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new Student { Name = "Damon", Age = 28, Grade = "B" },
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new Student { Name = "Echo", Age = 18, Grade = "C" }
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};
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// 使用SELECT创建一个新的匿名类，并输出为集合，一般配合Where使用
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var result1 = students.Select(student =>
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new
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{
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student.Name,
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student.Age
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});
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// 使用SELECT创建一个新的指定类，并输出为集合
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var result2 = students.Select(student => new StudentDto()
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{
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StudentName = student.Name,
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StudentAge = student.Age
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});
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```
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### ORDERBY
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> 排序
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```cs
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// 示例数据源
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List<Student> students = new List<Student>
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{
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new Student { Name = "Alice", Age = 25, Grade = "A" },
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new Student { Name = "Bob", Age = 30, Grade = "B" },
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new Student { Name = "Barry", Age = 35, Grade = "C" },
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new Student { Name = "Charlie", Age = 22, Grade = "A" },
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new Student { Name = "David", Age = 21, Grade = "C" },
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new Student { Name = "Damon", Age = 28, Grade = "B" },
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new Student { Name = "Echo", Age = 18, Grade = "C" }
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};
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// 使用LINQ的OrderBy进行排序
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var result1 = students.OrderBy(student => student.Age);
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// 使用LINQ的OrderByDescending进行降序排序
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var result2 = students.OrderByDescending(student => student.Age);
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```
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<!--rehype:className=wrap-text-->
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### JOIN
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<!--rehype:wrap-class=col-span-3-->
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> `Join`：用于执行内连接操作，它会返回两个数据源中满足连接条件的元素的交集
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>
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> `GroupJoin`：用于执行左外连接（left outer join）操作，它会返回左边数据源的所有元素，以及每个元素所匹配的右边数据源的元素组成的集合。(嵌套)
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```cs
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// 示例数据源
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List<Department> departments = new List<Department>
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{
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new Department { ID = 1, Name = "HR" },
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new Department { ID = 2, Name = "IT" }
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};
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// 示例数据源
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List<Employee> employees = new List<Employee>
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{
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new Employee { DepartmentID = 1, Name = "Alice" },
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new Employee { DepartmentID = 2, Name = "Bob" },
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new Employee { DepartmentID = 1, Name = "Charlie" },
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new Employee { DepartmentID = 3, Name = "David" }
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};
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// 使用Join，将部门和员工相结合，获取部门名称和员工名称的集合
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var joinQuery = departments.Join(employees,
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department => department.ID, employee => employee.DepartmentID,
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(department, employee) => new { Department = department.Name, Employee = employee.Name }
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);
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// 使用GroupJoin，将部门和员工相结合，返回所有的部门，并返回每个部门相关联的员工集合(嵌套)
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var groupJoinQuery = departments.GroupJoin(employees,
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department => department.ID, employee => employee.DepartmentID,
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(department, employeeGroup) => new
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{
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Department = department.Name,
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Employees = employeeGroup.Select(e => e.Name).ToList()
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}
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);
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```
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### 结果转换
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<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
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```cs
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// ToList(): 将结果转换为List集合。
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List<Student> resultList = result.ToList();
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// ToDictionary(): 将结果转换为Dictionary字典。
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Dictionary<string, int> resultDictionary = students
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.ToDictionary(student => student.Name, student => student.Age);
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// ToArray(): 将结果转换为数组。
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Student[] resultArray = result.ToArray();
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// First(): 获取结果中的第一个元素。
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Student firstStudent = result.First();
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// FirstOrDefault(): 获取结果中的第一个元素，如果结果为空则返回默认值。
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Student firstStudent = result.FirstOrDefault();
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```
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### 自定义扩展方法
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```cs
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public static class CustomExtensions
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{
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public static IEnumerable<T> CustomFilter<T>(this IEnumerable<T> source, Func<T, bool> predicate)
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{
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foreach (var item in source)
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{
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if (predicate(item))
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{
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yield return item;
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}
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}
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}
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}
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// 使用自定义扩展方法
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var filteredData = students.CustomFilter(s => s.Age > 20);
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```
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<!--rehype:className=wrap-text-->
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### 示例
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<!--rehype:wrap-class=col-span-3&style=background:#e91e63;-->
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> 假设有一个包含学生信息的列表，每个学生有姓名、年龄和成绩。使用LINQ查询来选择年龄大于20岁的学生，然后按照他们的成绩进行分组，并选择每个分组中年龄最小的学生的姓名。
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```cs
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// 示例数据源
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List<Student> students = new List<Student>
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{
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new Student { Name = "Alice", Age = 25, Grade = "A" },
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new Student { Name = "Bob", Age = 30, Grade = "B" },
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new Student { Name = "Barry", Age = 35, Grade = "C" },
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new Student { Name = "Charlie", Age = 22, Grade = "A" },
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new Student { Name = "David", Age = 21, Grade = "C" },
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new Student { Name = "Damon", Age = 28, Grade = "B" },
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new Student { Name = "Echo", Age = 18, Grade = "C" }
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};
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// 使用LINQ进行查询
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var result = students
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.Where(student => student.Age > 20) // WHERE: 选择年龄大于20的学生
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.GroupBy(student => student.Grade) // GROUP BY: 按成绩分组
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.Select(group => group.OrderBy(student => student.Age).First().Name) // SELECT: 选择每个分组中年龄最小的学生的姓名
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.ToList(); //转换为List<Student>()
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//输出结果
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["Charlie","Damon","David"]
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```
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语法糖
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语法糖
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----
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----
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