C# 0 The Complete Reference

let

 statement, 

x

 is set equal to the average of the values in 

nums

. This value is 

obtained by calling 

Average( )

 on 

nums

.

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P a r t   I :  

T h e   C #   L a n g u a g e

Defe

rred vs. Immediate Query Execution

In LINQ, queries have two different modes of execution: immediate and deferred. As 

explained early in this chapter, a query defines a set of rules that are not actually executed 

until a 

foreach

 statement executes. This is called 

deferred execution.

However, if you use one of the extension methods that produces a nonsequence result, 

then the query must be executed to obtain that result. For example, consider the 

Count( )

method. In order for 

Count( )

 to return the number of elements in the sequence, the query 

must be executed, and this is done automatically when 

Count( )

 is called. In this case, 

immediate execution

 takes place, with the query being executed automatically in order to 

obtain the result. Therefore, even though you don’t explicitly use the query in a 

foreach

loop, the query is still executed.

Here is a simple example. It obtains the number of positive elements in the sequence.

// Use immediate execution.

using System;

using System.Linq;

class ImmediateExec {

  static void Main() {

    int[] nums =  { 1, -2, 3, 0, -4, 5 };

    // Create a query that obtains the number of positive

    // values in nums.

    int len = (from n in nums

               where n > 0

               select n).Count();

    Console.WriteLine("The number of positive values in nums: " + len);

  }

}

The output is

The number of positive values in nums: 3

In the program, notice that no explicit 

foreach

 loop is specified. Instead, the query 

automatically executes because of the call to 

Count( )

.

As a point of interest, the query in the preceding program could also have been written 

like this:

var posNums = from n in nums

              where n > 0

              select n;

int len = posNums.Count(); // query executes here

In this case, 

Count( )

 is called on the query variable. At that point, the query is executed to 

obtain the count.

Two other methods that cause immediate execution of a query are 

ToArray( )

 and 

ToList( )

. Both are extension methods defined by 

Enumerable

.

ToArray( )

 returns the results 

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