101 LINQ Samples: Query Execution

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原文链接：http://www.cnblogs.com/zjz008/archive/2011/03/13/1982979.html

本文深入探讨了LINQ中延迟执行和即时执行的概念，通过示例展示了查询如何在枚举时才执行，以及如何通过使用ToList()等方法立即执行查询并缓存结果。此外，还介绍了如何利用延迟执行特性，在数据变化后重新使用同一查询，以获取更新后的结果。

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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

转载于:https://www.cnblogs.com/zjz008/archive/2011/03/13/1982979.html