一、列表子项提取
class Program
{static void Main(){var students = new List<Student>{new Student { Name = "Alice", Courses = new List<string> { "Math", "Science" } },new Student { Name = "Bob", Courses = new List<string> { "English", "History" } },new Student { Name = "Charlie", Courses = new List<string> { "Art", "Music" } }};//注意：x.Courses的类型是List<string>，SelectMany提取后的结果类型不变，一样是List<string>List<string> course_list = students.SelectMany(x => x.Courses).ToList();//["Math", "Science", "English", "History", "Art", "Music"]}
}二、双重for循环
class Program
{static void Main(){var students = new List<string> { "Alice", "Bob", "Charlie" };var grades = new List<int> { 90, 80, 85 };//x是students的子项，y是grades的子项，第二入参是x和y，出参可以自定义类型var result = students.SelectMany(x => grades, (x, y) => new { Name = x, Score = y });// 主列表.SelectMany(x => 次列表, 入参为x和y出参可自定义的委托)foreach (var item in result){Console.WriteLine($"{item.Name} ：{item.Score}");}/*Alice ：90Alice ：80Alice ：85Bob ：90Bob ：80Bob ：85Charlie ：90Charlie ：80Charlie ：85*/}
}三、笛卡尔积（扩展）
C#高级：结合Linq的SelectMany方法实现笛卡尔积效果_c# selectmany-CSDN博客
