Deferred Execution

The following sample shows how query execution is deferred until the query is enumerated at a foreach statement.

public void Linq99(){    // Sequence operators form first-class queries that    // are not executed until you enumerate over them.     int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };     int i = 0;    var q =        from n in numbers        select ++i;     // Note, the local variable 'i' is not incremented    // until each element is evaluated (as a side-effect):    foreach (var v in q)    {        Console.WriteLine("v = {0}, i = {1}", v, i);    }}

Result

v = 1, i = 1

v = 2, i = 2

v = 3, i = 3

v = 4, i = 4

v = 5, i = 5

v = 6, i = 6

v = 7, i = 7

v = 8, i = 8

v = 9, i = 9

v = 10, i = 10

Immediate Execution

The following sample shows how queries can be executed immediately with operators such as ToList().

public void Linq100(){    // Methods like ToList() cause the query to be    // executed immediately, caching the results.     int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };     int i = 0;    var q = (        from n in numbers        select ++i)        .ToList();     // The local variable i has already been fully    // incremented before we iterate the results:    foreach (var v in q)    {        Console.WriteLine("v = {0}, i = {1}", v, i);    }}

Result

v = 1, i = 10

v = 2, i = 10

v = 3, i = 10

v = 4, i = 10

v = 5, i = 10

v = 6, i = 10

v = 7, i = 10

v = 8, i = 10

v = 9, i = 10

v = 10, i = 10

Query Reuse

The following sample shows how, because of deferred execution, queries can be used again after data changes and will then operate on the new data.

public void Linq101(){    // Deferred execution lets us define a query once    // and then reuse it later after data changes.     int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };    var lowNumbers =        from n in numbers        where n <= 3        select n;     Console.WriteLine("First run numbers <= 3:");    foreach (int n in lowNumbers)    {        Console.WriteLine(n);    }     for (int i = 0; i < 10; i++)    {        numbers[i] = -numbers[i];    }     // During this second run, the same query object,    // lowNumbers, will be iterating over the new state    // of numbers[], producing different results:    Console.WriteLine("Second run numbers <= 3:");    foreach (int n in lowNumbers)    {        Console.WriteLine(n);    }}

Result

First run numbers <= 3:

1

3

2

0

Second run numbers <= 3:

-5

-4

-1

-3

-9

-8

-6

-7

-2

0