using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var userList = 

            User.Factory.Generate(100)

            .OnlyActive()

            .YoungerThan(20);

        userList.Print(); // print is also an extension method

        Console.WriteLine("-------");

        var exists = userList

            .GetById(30)

            .IsNotNull();

        Console.WriteLine("exists: " + exists.ToString());

        Console.WriteLine("-------");

        userList

            .GetById(3)

            .IfNotNull(user=> Console.WriteLine( user.Name + " found!"));

    }

}

​

public static class Extensions {

    public static void Print(this IEnumerable<User> list){

        list

            .ToList()

            .ForEach(i=> Console.WriteLine(i.Name));

    }

    public static IEnumerable<User> OnlyActive(this IEnumerable<User> list) {

        return list.Where(i=> i.Status == User.StatusCode.Active);

    }

    public static IEnumerable<User> OlderThan(this IEnumerable<User> list, int age) {

        return list.Where(i=> i.Age > age);

    }

​

    public static IEnumerable<User> YoungerThan(this IEnumerable<User> list, int age) {

        return list.Where(i=> i.Age < age);

    }

    public static User GetById(this IEnumerable<User> list, int Id) {

        return list.SingleOrDefault(i=> i.Id == Id);

    }

}

​

public static class OtherExtensions{

    public static bool IsNull(this object obj){

        return obj == null;

    }

    public static bool IsNotNull(this object obj){

        return !obj.IsNull();

    }

    public static void IfNotNull<T>(this T obj, Action<T> action){

        if(obj.IsNotNull()){

            action(obj);

        }

    }

}

​

public sealed class User {

    public int Id {set;get;}

    public string Name {set;get;}

    public int Age {set;get;}

    public StatusCode Status {set;get;}

    public User(int id){

        Id = id;

        Name = "name_" + id.ToString();

        Age = id; // let's assume id equals to age for simplicity

        Status = getCode(id);

    }

    public enum StatusCode {

        Active,

        InActive,

        Suspended

    }

    static StatusCode getCode(int id) {

        var m = id % 3;

        switch(m){

            case 0: return StatusCode.Active;

            case 1: return StatusCode.InActive;

            case 2: return StatusCode.Suspended;

            default: throw new Exception();

        }

    }

    public static class Factory {

        public static List<User> Generate(int count){

            return Enumerable

                .Range(1,count)

                .Select(i=> new User(i))

                .ToList()

                ;

        }

    }

}