......

......

static void Main(string[] args)
        {

            ArrayList al = new ArrayList();
            al.Add(new Person("Chiwa",25));
            al.Add(new Person("Chiwa2",26));
            al.Add(new Person("Chiwa3",27));
            al.Add(new Person("Chiwa4",28));

            var person =
                from Person p in al
                select p;

            foreach(var p in person)
            {
                Console.WriteLine(p.name+"        "+p.age);
                p.echo();
            }

            Console.ReadLine();
        }//================================//

    }//end class

    //==================================//
    class Person
    {
        public string name;
        public int age;

        public Person(String n, int a)
        {
            this.name = n;
            this.age = a;
        }
        public void echo()
        {
            Console.WriteLine("Echo ==> "+this.name);
        }//========================//
    }
    //======================================//

ใส่ condition ก็ได้นะเช่น

var person =
                from Person p in al
                where p.age <= 26 && ............
                select p;

แล้วถ้าผมจะหาอายุเฉลี่ยล่ะ ก็แค่นี้ครับ

   var av = person.Average(p => p.age);
            Console.WriteLine("average age is {0} .", av);

ลองใช้ List ดู ก็ง่ายดี

 Console.WriteLine("=================USE LIST================");
            List<Person> Persons = new List<Person>();
            Persons.Add(new Person("Chiwa", 25));
            Persons.Add(new Person("Chiwa2", 26));
            Persons.Add(new Person("Chiwa3", 27));
            Persons.Add(new Person("Chiwa4", 28));

            var avr = Persons.Where(p => p.age >= 27).Sum(p => p.age);
            Console.WriteLine("Sum age is {0} .", avr);
            Console.ReadLine();