请教一个集合排序的问题

 public class Student

    {

        public string First { get; set; }

        public string Last { get; set; }

        public int ID { get; set; }

    }



    public static List<Student> GetStudents()

    {

        // Use a collection initializer to create the data source. Note that each element

        //  in the list contains an inner sequence of scores.

        List<Student> students = new List<Student>

        {

           new Student {First="Svetlana", Last="Omelchenko", ID=111},

           new Student {First="Claire", Last="O'Donnell", ID=112},

           new Student {First="Sven", Last="Mortensen", ID=113},

           new Student {First="Cesar", Last="Garcia", ID=114},

           new Student {First="Debra", Last="Garcia", ID=115} 

        };



        return students;



    }

GetStudents().OrderBy(u => u.First)
GetStudents().OrderBy(u => u.Last)
GetStudents().OrderBy(u => u.ID)

可以排序，如果被排序的字段是个变量，应该怎么写呢？
如 string field="First";
GetStudents().OrderBy(u => u.?)