LINQ Deferred Execution

Introduction

The following example shows how query execution is deferred until the results is enumerated (requested in other words.)

static void TryLinq()
{
int i = 0;
int[] numbers = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// fake values for the query only (10 values)var result = from n in numbers select Increment(ref i);
Console.WriteLine("After query i = {0}", i); // i still 0Console.WriteLine();
Console.WriteLine("Enumerating results:");
foreach (var v in result)
{
Console.WriteLine("v = {0},ti = {1}", v, i);
// i is incremented every loop}
Console.WriteLine("Press any key to continue . . .");
Console.ReadKey(true);
}

// Result:-
// After query i = 0

// Enumerating results:
// v = 1,  i = 1
// v = 2,  i = 2
// .............
// v = 9,  i = 9
// v = 10, i = 10

// What you get?
// Deferred-Execution / Lazy-Execution
// - Query doesn't execute until you
//   begin retrieving it's results.
// - Every time you try get a value
//   the query executes on this value only.

Next is an example shows how you can immediate-execute the query.

static void TryLinq()
{
int i = 0;
int[] numbers = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
// fake values for the query only (10 values)var result =
(from n in numbers select Increment(ref i)).ToList();
// The last call tries to get the value immediately.Console.WriteLine("After query i = {0}", i); // i is 10Console.WriteLine();
Console.WriteLine("Enumerating results:");
foreach (var v in result)
{
Console.WriteLine("v = {0},ti = {1}", v, i);
// i still 10 every loop}
Console.WriteLine("Press any key to continue . . .");
Console.ReadKey(true);

// Result:-// After query i = 10// Enumerating results:// v = 1,  i = 10// v = 2,  i = 10// .............// v = 9,  i = 10// v = 10, i = 10// What you get?// Deferred-Execution / Lazy-Execution// - Query doesn't execute until you//   begin retrieving it's results.// - Every time you try get a value the//   query executes on this value only.// - You can immediate-execute the query by//   calling some conversation methods like//   ToList or ToQuery.}

static int Increment(ref int i) { return ++i; }

What you get?

Deferred-Execution / Lazy-Execution