2重foreachをLINQを使って

public class Student
{
    public string Name;
}

public class Class
{
    public Student[] Students;
}

public class School
{
    public Class[] Classes;
}

Schoolから全Studentの名前のリストを作りたい場合

foreachなら

var names = new List<string>();
foreach (var @class in school.Classes)
{
    foreach (var student in @class.Students)
    {
        names.Add(student.Name);
    }
}

LINQを使うと

var names = school.Classes
    .SelectMany(o => o.Students)
    .Select(o => o.Name)
    .ToList();