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延迟执行的经典例子：

我们用 select ++i 就可以看到在foreach 时候，查询才被执行。

public static void Linq99()
{
    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;
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
}

输出结果：

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

foreach每一个遍历的时候，select出来的值和当前i的值都是一样的。

立即执行的经典例子：

public static void Linq99()
{
    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();
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
}

执行结果：

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

这个例子的代码跟上面延迟执行的例子代码唯一的差别在于多了一个.ToList();
这也可以证明我们之前提到的原则：

只有到用的时候才会去执行查询

由于 .ToList(); 的存在，在这里就要用到了，所以在这里就执行了查询，而不是在foreach中执行查询。注意，这时候出来的结果是一个数组了．参看后面的几个例子．

执行的一个特殊情况：重复执行

请看下面例子：

查询出一个int数组中小于3的数字。

下面例子中在第一次查询后，对数据源作了修改，然后再作第二次查询，我们可以看到第二次我们不需要再作

lowNumbers = from n in numbers where n <= 3 select n; 这样的定义，而是直接使用    foreach (int n in lowNumbers)。另外这两次的返回结果是不同的，因为我们
在第一次查询后，对数据源作了修改。

public static void Linq101()
{
    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];

    Console.WriteLine(“Second run numbers <= 3:”);
    foreach (int n in lowNumbers)
        Console.WriteLine(n);
}

输出结果：

First run numbers <= 3:
1
3
2
0
Second run numbers <= 3:
-5
-4
-1
-3
-9
-8
-6
-7
-2
0

下面我们再来看几个例子，加深对查询执行的理解：

重复查询的再一个例子：

public static void Linq102()
{
    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;
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
}

执行结果：

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
v = 11, i = 11
v = 12, i = 12
v = 13, i = 13
v = 14, i = 14
v = 15, i = 15
v = 16, i = 16
v = 17, i = 17
v = 18, i = 18
v = 19, i = 19
v = 20, i = 20

只执行一次的立即查询：

public static void Linq102()
{
    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();
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
    foreach (var v in q)
        Console.WriteLine(“v = {0}, i = {1}”, v, i);
}

执行结果：

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
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