c# - Expression is not filtering immediately -

please see sample below. linq expression asking list of student objects _age in between 15 , 20. when see varlist in debugger, shows 5 entries. when print varlist, prints 1 entry, correct. after printing, debugger shows 5 entries in varlist. behavior documented where, please let me know. should there lot of entries in list, millions, linq expression filter items.

public class student {     public int studentid { get; set; }     public string studentname { get; set; }     public int age { get; set; } }  static void main(string[] args) {     ilist<student> studentlist = new list<student>     {          new student { studentid = 1, studentname = "john", age = 13 },          new student { studentid = 2, studentname = "moin", age = 21 },          new student { studentid = 3, studentname = "bill", age = 18 },          new student { studentid = 4, studentname = "ram", age = 20 },          new student { studentid = 5, studentname = "ron", age = 15 }    };      ienumerable<student> varlist = rv in studentlist                                    rv.age > 15 && rv.age < 20                                    select rv;     foreach (student x in varlist)     {         console.writeline("{0} {1}", x.age, x.studentname);     } } 

this how deferred execution works linq. linq query has expression tree behind, can @ linq query data structure. query not execute until request data, when expression compiled, happens when enumerating through it.

you can read more here:

http://blogs.msdn.com/b/charlie/archive/2007/12/09/deferred-execution.aspx

ienumerable<customer> query = customer in db.customers  << query           customer.city == "paris" << not execute         select customer;               << here    foreach (var customer in query) << query executes here 

no matter using ienumerable or iqueryable represent query variable, both going use deferred execution:

returning ienumerable<t> vs. iqueryable<t>