C# LINQ SelectMany方法详解

// 基础示例
var lists = new List<List<int>> {
    new List<int> { 1, 2, 3 },
    new List<int> { 4, 5, 6 }
};

var flattened = lists.SelectMany(x => x);
// 结果: [1, 2, 3, 4, 5, 6]

var result = lists.SelectMany((list, index) => 
    list.Select(item => $"列表{index}: {item}"));

public class Student
{
    public string Name { get; set; }
    public List<Course> Courses { get; set; }
}

// 获取所有学生的所有课程
var allCourses = students.SelectMany(s => s.Courses);

// 带学生信息的课程列表
var studentCourses = students.SelectMany(
    student => student.Courses,
    (student, course) => new { 
        StudentName = student.Name, 
        CourseName = course.Name 
    }
);

string[] words = { "Hello", "World" };
var letters = words.SelectMany(word => word.ToLower());
// 结果: ['h','e','l','l','o','w','o','r','l','d']

// 方法语法
var result = students.SelectMany(s => s.Courses);

// 等价的查询语法
var result = from student in students
            from course in student.Courses
            select course;

var result = students.SelectMany(
    student => student.Courses.Where(c => c.Credits > 3),
    (student, course) => new {
        Student = student.Name,
        Course = course.Name,
        Credits = course.Credits
    });

var departments = new List<Department>();
var result = departments
    .SelectMany(d => d.Teams)
    .SelectMany(t => t.Employees);

// 处理可能为null的集合
var result = students.SelectMany(s => 
    s.Courses ?? Enumerable.Empty<Course>());