C#学习记录7——LINQ

LINQ(Language-INtegrated Query)语言集成查询，可以帮助我们将查询功能添加到C#中。可用于数据库等检索。

声明Query十分简单，语法上与SQL语言十分相似，只是有少许的区别

关于LINQ，在msdn上有101 Samples十分详细介绍解释LINQ如何使用在具体工程中。链接：https://code.msdn.microsoft.com/101-linq-samples-3fb9811b

一、Query Execution 查询的执行

介绍了Query执行中的特点

1.Query的延迟执行

            public void Linq99()
            {

                // Queries 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 simpleQuery =
                    from num in numbers
                    select ++i;

                // The local variable 'i' is not incremented
                // until the query is executed in the foreach loop.
                Console.WriteLine("The current value of i is {0}", i); //i is still zero

                foreach (var item in simpleQuery)
                {
                    Console.WriteLine("v = {0}, i = {1}", item, i);       
                }
            }        

8-10行声明了一个qurey。由于没有where，所以numbers中所有元素都符合要求，每次找到符合的元素，++i

14行的输出语句，i仍旧是0。原因是声明query不会立刻执行

16行foreach语句才是真正执行query语句的位置

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

2.一个query多次复用（Query Reuse）

            public void Linq101()
            {

                // Deferred execution lets us define a query once
                // and then reuse it later in various ways.
                int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
                var lowNumbers =
                    from num in numbers
                    where num <= 3
                    select num;

                Console.WriteLine("First run numbers <= 3:");
                foreach (int n in lowNumbers)
                {
                    Console.WriteLine(n);
                }

                // Query the original query.
                var lowEvenNumbers =
                    from num in lowNumbers
                    where num % 2 == 0
                    select num;

                Console.WriteLine("Run lowEvenNumbers query:");
                foreach (int n in lowEvenNumbers)
                {
                    Console.WriteLine(n);
                }

                // Modify the source data.
                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);
                }
                Console.WriteLine("Second Run lowEvenNumbers query:");
                foreach (int n in lowEvenNumbers)
                {
                    Console.WriteLine(n);
                }
            }

首先注意到9，21行出现的where语句会根据一定条件选择结果
在13行和40行中两次使用的是同一个query语句。两次查询之间，将numbers中的所有数字全部取相反数。结果也随之改变。

可以说，创建query类似于创建了sql里面的一个view（视图），可以通过视图来查询制定条件的结果。

结果

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

3.强制立即执行查询

            public void Linq100()
            {

                // Methods like ToList(), Max(), and Count() cause the query to be
                // executed immediately.            
                int[] numbers = new int[] { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

                int i = 0;
                var immediateQuery = (
                    from num in numbers
                    select ++i)
                    .ToList();

                Console.WriteLine("The current value of i is {0}", i); //i has been incremented

                foreach (var item in immediateQuery)
                {
                    Console.WriteLine("v = {0}, i = {1}", item, i);
                }
            }

与例1不同的在第12行，查询后使用了ToList()强制立刻执行当前的query查询。结合例2可以理解为，这里可以利于保存数据库的某个临时缓存结果，查看中期的结果。

结果

The current value of i is 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

二、Query排序

            public void Linq28()
            {
                string[] words = { "cherry", "apple", "blueberry" };

                var sortedWords =
                    from word in words
                    orderby word
                    select word;

                Console.WriteLine("The sorted list of words:");
                foreach (var w in sortedWords)
                {
                    Console.WriteLine(w);
                }
            }

7行orderby默认按照字符串的字母顺序从小到大排序
类似的还有：w.Length按照字符串长度从小到大排序

结果

The sorted list of words:
apple
blueberry
cherry

msdn101samples将其分成了几大部分，下载后的每个部分都有description文件，详细讲述了其中的例子，强烈推荐！