C# 2008 Programmer's Reference

65
33
23

Because the output for the second foreach loop is different from the first, the program effectively proves that the query is not executed until it is accessed.

Deferred execution works regardless of whether you are using the query or method syntax.

Forced Immediate Query Execution

One way to force an immediate execution of the query is to explicitly convert the query result into a List object. For example, the following query converts the result to a List object:

var oddNums = nums.Where
 (n => n % 2 == 1).OrderByDescending(n => n).ToList ();

In this case, the query is executed immediately, as proven by the following program and its output: using System;

using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace ConsoleApplication5 {
 class Program {
  static void Main(string[] args) {
   int[] nums = {
    12, 34, 10, 3, 45, 6, 90, 22, 87, 49, 13, 32
   };
   var oddNums = nums.Where
    (n => n % 2 == 1).OrderByDescending(n => n).ToList ();
   Console.WriteLine('First execution');
   Console.WriteLine('---------------');
   foreach (int n in oddNums) Console.WriteLine(n);
   //---add 20 to each number in the array---
   for (int i = 0; i < 11; i++) nums[i] += 20;
   Console.WriteLine('Second execution');
   Console.WriteLine('----------------');
   foreach (int n in oddNums) Console.WriteLine(n);
   Console.ReadLine();
  }
 }
}

Here's the program's output:

First execution
---------------
87
49
45
13
3
Second execution
87
49
45
13
3

The output of the first and second execution is the same, proving that the query is executed immediately after it's defined.

To force a LINQ query to execute immediately, you can use aggregate functions so that the query must iterate over the elements at once. An aggregate function takes a collection of values and returns a scalar value.

Aggregate functions are discussed in more detail later in this chapter.

Following is an example that uses the Count() aggregate function. The program selects all the odd numbers from an array and then counts the total number of odd numbers. Each number is then multiplied by two (which makes them all become even numbers).

static void Main(string[] args) {
 int[] nums = {
  12, 34, 10, 3, 45, 6, 90, 22, 87, 49, 13, 32
 };
 var oddNumsCount = nums.Where
  (n => n % 2 == 1).OrderByDescending(n => n).Count ();
 Console.WriteLine('First execution');
 Console.WriteLine('---------------');
 Console.WriteLine('Count: {0}', oddNumsCount);
 //---add 20 to each number in the array---
 for (int i = 0; i < 11; i++)
  nums[i] *= 2; //---all number should now be even---
 Console.WriteLine('Second execution');
 Console.WriteLine('----------------');
 Console.WriteLine('Count: {0}', oddNumsCount);
 Console.ReadLine();
}

The output shows that once the query is executed, its value does not change:

First execution
---------------
Count: 5
Second execution
----------------
Count: 5

LINQ and Anonymous Types

Although Chapter 4 explored anonymous types and how they allow you to define data types without having