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#
This commit is contained in:
Willxup
261
docs/cs.md
261
docs/cs.md
@@ -1065,6 +1065,267 @@ bool hasKey2 = hashTable.ContainsKey("key2");
|
||||
string valueOfKey2 = (string)hashTable["key2"];
|
||||
```
|
||||
|
||||
|
||||
|
||||
LINQ
|
||||
--------
|
||||
|
||||
> 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。
|
||||
|
||||
|
||||
|
||||
### FROM
|
||||
|
||||
<!--rehype:wrap-class=col-span-3-->
|
||||
|
||||
> 任何数据源，包括对象集合、数据库、XML等。
|
||||
|
||||
|
||||
|
||||
### WHERE
|
||||
|
||||
<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
|
||||
|
||||
> 条件查询
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Student> students = new List<Student>
|
||||
{
|
||||
new Student { Name = "Alice", Age = 25, Grade = "A" },
|
||||
new Student { Name = "Bob", Age = 30, Grade = "B" },
|
||||
new Student { Name = "Barry", Age = 35, Grade = "C" },
|
||||
new Student { Name = "Charlie", Age = 22, Grade = "A" },
|
||||
new Student { Name = "David", Age = 21, Grade = "C" },
|
||||
new Student { Name = "Damon", Age = 28, Grade = "B" },
|
||||
new Student { Name = "Echo", Age = 18, Grade = "C" }
|
||||
};
|
||||
|
||||
// 使用WHERE筛选出成绩为A的学生
|
||||
var result1 = students.Where(student => student.Grade = "A");
|
||||
|
||||
// 使用WHERE筛选出年龄大于20的学生
|
||||
var result2 = students.Where(student => student.Age > 20);
|
||||
|
||||
// 使用WHERE筛选出名字包含'D'的学生
|
||||
var result3 = students.Where(student => student.Name.Contains("D"));
|
||||
|
||||
// 使用WHERE筛选出名字为'Bob'和'Echo'的学生
|
||||
List<string> nameList = new() { "Bob", "Echo" };
|
||||
var result4 = students.Where(student => nameList.Contains(student.Name));
|
||||
```
|
||||
|
||||
|
||||
|
||||
### GROUPBY
|
||||
|
||||
> 分组查询
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Student> students = new List<Student>
|
||||
{
|
||||
new Student { Name = "Alice", Age = 25, Grade = "A" },
|
||||
new Student { Name = "Bob", Age = 30, Grade = "B" },
|
||||
new Student { Name = "Barry", Age = 35, Grade = "C" },
|
||||
new Student { Name = "Charlie", Age = 22, Grade = "A" },
|
||||
new Student { Name = "David", Age = 21, Grade = "C" },
|
||||
new Student { Name = "Damon", Age = 28, Grade = "B" },
|
||||
new Student { Name = "Echo", Age = 18, Grade = "C" }
|
||||
};
|
||||
|
||||
// 使用GROUP BY按成绩进行分组查询
|
||||
var groupedByGrade = students.GroupBy(student => student.Grade);
|
||||
```
|
||||
|
||||
<!--rehype:className=wrap-text-->
|
||||
|
||||
|
||||
|
||||
### SELECT
|
||||
|
||||
<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
|
||||
|
||||
> 结果查询
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Student> students = new List<Student>
|
||||
{
|
||||
new Student { Name = "Alice", Age = 25, Grade = "A" },
|
||||
new Student { Name = "Bob", Age = 30, Grade = "B" },
|
||||
new Student { Name = "Barry", Age = 35, Grade = "C" },
|
||||
new Student { Name = "Charlie", Age = 22, Grade = "A" },
|
||||
new Student { Name = "David", Age = 21, Grade = "C" },
|
||||
new Student { Name = "Damon", Age = 28, Grade = "B" },
|
||||
new Student { Name = "Echo", Age = 18, Grade = "C" }
|
||||
};
|
||||
|
||||
// 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
|
||||
var result1 = students.Select(student =>
|
||||
new
|
||||
{
|
||||
student.Name,
|
||||
student.Age
|
||||
});
|
||||
|
||||
// 使用SELECT创建一个新的指定类，并输出为集合
|
||||
var result2 = students.Select(student => new StudentDto()
|
||||
{
|
||||
StudentName = student.Name,
|
||||
StudentAge = student.Age
|
||||
});
|
||||
```
|
||||
|
||||
|
||||
|
||||
### ORDERBY
|
||||
|
||||
> 排序
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Student> students = new List<Student>
|
||||
{
|
||||
new Student { Name = "Alice", Age = 25, Grade = "A" },
|
||||
new Student { Name = "Bob", Age = 30, Grade = "B" },
|
||||
new Student { Name = "Barry", Age = 35, Grade = "C" },
|
||||
new Student { Name = "Charlie", Age = 22, Grade = "A" },
|
||||
new Student { Name = "David", Age = 21, Grade = "C" },
|
||||
new Student { Name = "Damon", Age = 28, Grade = "B" },
|
||||
new Student { Name = "Echo", Age = 18, Grade = "C" }
|
||||
};
|
||||
|
||||
// 使用LINQ的OrderBy进行排序
|
||||
var result1 = students.OrderBy(student => student.Age);
|
||||
|
||||
// 使用LINQ的OrderByDescending进行降序排序
|
||||
var result2 = students.OrderByDescending(student => student.Age);
|
||||
```
|
||||
|
||||
<!--rehype:className=wrap-text-->
|
||||
|
||||
|
||||
|
||||
### JOIN
|
||||
|
||||
<!--rehype:wrap-class=col-span-3-->
|
||||
|
||||
> `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
|
||||
>
|
||||
> `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)
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Department> departments = new List<Department>
|
||||
{
|
||||
new Department { ID = 1, Name = "HR" },
|
||||
new Department { ID = 2, Name = "IT" }
|
||||
};
|
||||
// 示例数据源
|
||||
List<Employee> employees = new List<Employee>
|
||||
{
|
||||
new Employee { DepartmentID = 1, Name = "Alice" },
|
||||
new Employee { DepartmentID = 2, Name = "Bob" },
|
||||
new Employee { DepartmentID = 1, Name = "Charlie" },
|
||||
new Employee { DepartmentID = 3, Name = "David" }
|
||||
};
|
||||
|
||||
// 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
|
||||
var joinQuery = departments.Join(employees,
|
||||
department => department.ID, employee => employee.DepartmentID,
|
||||
(department, employee) => new { Department = department.Name, Employee = employee.Name }
|
||||
);
|
||||
|
||||
// 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)
|
||||
var groupJoinQuery = departments.GroupJoin(employees,
|
||||
department => department.ID, employee => employee.DepartmentID,
|
||||
(department, employeeGroup) => new
|
||||
{
|
||||
Department = department.Name,
|
||||
Employees = employeeGroup.Select(e => e.Name).ToList()
|
||||
}
|
||||
);
|
||||
```
|
||||
|
||||
|
||||
|
||||
### 结果转换
|
||||
|
||||
<!--rehype:wrap-class=col-span-2&style=background:#e91e63;-->
|
||||
|
||||
```cs
|
||||
// ToList(): 将结果转换为List集合。
|
||||
List<Student> resultList = result.ToList();
|
||||
|
||||
// ToDictionary(): 将结果转换为Dictionary字典。
|
||||
Dictionary<string, int> resultDictionary = students
|
||||
.ToDictionary(student => student.Name, student => student.Age);
|
||||
|
||||
// ToArray(): 将结果转换为数组。
|
||||
Student[] resultArray = result.ToArray();
|
||||
|
||||
// First(): 获取结果中的第一个元素。
|
||||
Student firstStudent = result.First();
|
||||
|
||||
// FirstOrDefault(): 获取结果中的第一个元素，如果结果为空则返回默认值。
|
||||
Student firstStudent = result.FirstOrDefault();
|
||||
```
|
||||
|
||||
|
||||
|
||||
### 自定义扩展方法
|
||||
|
||||
```cs
|
||||
public static class CustomExtensions
|
||||
{
|
||||
public static IEnumerable<T> CustomFilter<T>(this IEnumerable<T> source, Func<T, bool> predicate)
|
||||
{
|
||||
foreach (var item in source)
|
||||
{
|
||||
if (predicate(item))
|
||||
{
|
||||
yield return item;
|
||||
}
|
||||
}
|
||||
}
|
||||
}
|
||||
// 使用自定义扩展方法
|
||||
var filteredData = students.CustomFilter(s => s.Age > 20);
|
||||
```
|
||||
|
||||
<!--rehype:className=wrap-text-->
|
||||
|
||||
### 示例
|
||||
|
||||
<!--rehype:wrap-class=col-span-3&style=background:#e91e63;-->
|
||||
|
||||
> 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。
|
||||
|
||||
```cs
|
||||
// 示例数据源
|
||||
List<Student> students = new List<Student>
|
||||
{
|
||||
new Student { Name = "Alice", Age = 25, Grade = "A" },
|
||||
new Student { Name = "Bob", Age = 30, Grade = "B" },
|
||||
new Student { Name = "Barry", Age = 35, Grade = "C" },
|
||||
new Student { Name = "Charlie", Age = 22, Grade = "A" },
|
||||
new Student { Name = "David", Age = 21, Grade = "C" },
|
||||
new Student { Name = "Damon", Age = 28, Grade = "B" },
|
||||
new Student { Name = "Echo", Age = 18, Grade = "C" }
|
||||
};
|
||||
|
||||
// 使用LINQ进行查询
|
||||
var result = students
|
||||
.Where(student => student.Age > 20) // WHERE: 选择年龄大于20的学生
|
||||
.GroupBy(student => student.Grade) // GROUP BY: 按成绩分组
|
||||
.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
|
||||
.ToList(); //转换为List<Student>()
|
||||
|
||||
//输出结果
|
||||
["Charlie","Damon","David"]
|
||||
```
|
||||
|
||||
语法糖
|
||||
----
|
||||
|
||||
|
||||
Reference in New Issue
Block a user