LING 实战

1、语法

1、LINQ所处在的主要命名空间：System.Linq

2、LINQ处理的核心对象就是IEnumerable可枚举对象，包括泛型枚举。换句话说当你要处理的对象为IEnumerable类型对象时，即可使用LINQ。

     在没有经过其它处理的情况下将返回一个新的IEnumerable序列，另外LING含有“延迟加载”特性。

3、关键字（摘自MSDN）

from：           指定数据源和范围变量（类似于迭代变量）

where：         根据一个或多个由逻辑“与”和逻辑“或”运算符（&&或||）分隔的布尔表达式筛选源元素

select：指定当执行查询时返回的序列中的元素将具有的类型和形式 

group：按照指定的键值对查询结果进行分组

into：提供一个标识符，它可以充当对 join、group、select子句的结果的引用 

orderby：基于元素类型的默认比较器按升序或降序对查询结果进行排序

join：基于两个指定匹配条件之间的相等比较来关联两个数据源

let：引入一个用于存储查询表达式中的子表达式结果的范围变量

in：join 子句中的上下文关键字

on：join 子句中的上下文关键字

equals：join 子句中的上下文关键字

by：group 子句中的上下文关键字

ascending：orderby 子句中的上下文关键字

descending：orderby 子句中的上下文关键字

4、语法说明

每个LINQ语句都以from作为开头，以select作为结束，这点和T-SQL语法不一致。

样例：IEnumerable<T> nums = from n in nums where ... orderby ... select ...

2、案例

        static void Main(string[] args)
        {
            string[] words = { "hello", "wonderful", "linq", "beautiful", "world" };
            //查询出较短的单词
            var shortsWords = from word in words
                              where word.Length <= 5
                              select word;
            //结果：hello  ling  world 
            foreach (var word in shortsWords)
            {
                Console.WriteLine(word);
            }
            Console.ReadKey();

        static void Main(string[] args)
        {
            string[] words = { "hello", "wonderful", "linq", "beautiful", "world" };
            //按长度将单词分组
            var groups = from word in words
                         group word by word.Length into lengthGroups
                         orderby lengthGroups.Key descending
                         select new { Length = lengthGroups.Key,Words=lengthGroups };

            foreach (var group in groups)
            {
                Console.WriteLine("单词长度为：{0}",group.Length);
                foreach (string word in group.Words)
                {
                    Console.WriteLine("  " + word);
                }
            }
            Console.ReadKey();

        static void Main(string[] args)
        {
            List<Class> clist = new List<Class> { 
                new Class{ CId="C1",CName="Java" },
                new Class{ CId="C2",CName=".NET" },
                new Class{ CId="C3",CName="Test" }
            };
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //内联查询
            var query_1 = from s in slist
                          join c in clist on s.CId equals c.CId
                          select new { SId = s.SId, SName = s.SName, CName=c.CName };

            foreach (var q in query_1)
            {
                Console.WriteLine("{0}\t{1}\t{2}",q.SId,q.SName,q.CName);
            }
            Console.WriteLine("----------------------------");


            //左外联查询
            var query_2 = from s in slist
                          join c in clist on s.CId equals c.CId into g
                          from subpet in g.DefaultIfEmpty()
                          select new { SId = s.SId, SName = s.SName, CName= (subpet == null ? null : subpet.CName) };

            foreach (var q in query_2)
            {
                Console.WriteLine("{0}\t{1}\t{2}", q.SId, q.SName, q.CName);
            }

            Console.ReadKey();

        static void Main(string[] args)
        {
            Book[] books = new Book[] { 
                new Book("Java","张三",2012),
                new Book(".NET","李四",2012),
                new Book("Test","王五",2000)
            };
            XElement xml = new XElement("books",
                from book in books
                where book.Year == 2012
                select new XElement("book",
                    new XAttribute("title", book.Title),
                    new XElement("publisher", book.Publisher)
                )
             );
            Console.WriteLine(xml);

            Console.ReadKey();

3、Lambda 表达式 

入门示例：

        static void Main(string[] args)
        {
            var processes = Process.GetProcesses()
                            .Where(process => process.WorkingSet64 > 20*1024*1024)
                            .OrderByDescending(process => process.WorkingSet64)
                            .Select(process => new { process.Id, Name=process.ProcessName });

            foreach(var p in processes)
            {
                Console.WriteLine(p.Name);
            }
            Console.ReadKey();

        static void Main(string[] args)
        {    
            var processes = from process in Process.GetProcesses()
                            where process.WorkingSet64 > 20 * 1024 * 1024
                            orderby process.WorkingSet64 descending
                            select new { process.Id, Name = process.ProcessName };

            foreach(var p in processes)
            {
                Console.WriteLine(p.Name);
            }
            Console.ReadKey();

Func<T,TResult> 泛型委托类型：

        static void Main(string[] args)
        {
            Func<int, bool> isOdd = i => (i & 1) == 1;

            for (int i = 0; i < 10; i++)
            {
                if (isOdd(i))
                {
                    Console.WriteLine("{0} is odd", i);
                }
                else
                {
                    Console.WriteLine("{0} is even", i);
                }
            }
            Console.ReadKey();

示例：

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            var stus = slist.Where(stu => stu.SName.Contains("飞"))
                            .Select(stu => stu.SName);
            foreach (var s in stus)
            {
                Console.WriteLine(s);
            }
            Console.ReadKey();

标准查询操作符：（以下为常用的）

过滤：Where

投影：Select、SelectMany

分区：Skip、Take

连接：GroupJoin、Join

排序：OrderBy、OrderByDescending、ThenBy、ThenByDescending

分组：GroupBy

集合：Distinct

转换：ToArray、ToDictionary、ToList

生成：DefaultIfEmpty

聚集：Count、Max、Min、Sum

1、约束操作符 Where 

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //约束操作符 Where
            var stus = slist.Where(stu => stu.SName.Contains("飞"));
            //等价于
            var query = from stu in slist
                        where stu.SName.Contains("飞")
                        select stu;

2、投影操作符 Select

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //投影操作符 Select
            var stus = slist.Select(stu => stu.SName);
            //等价于
            var query = from stu in slist                        
                        select stu.SName;

3、操作符 Distinct 

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //操作符 Distinct
            var stus = slist.Select(stu => stu.CId)
                            .Distinct();

4、转换操作符 ToArray、ToList

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //转换操作符
            Student[] stuArr = slist.Where(stu => stu.SAge < 26)
                            .ToArray();
            List<Student> stuList = slist.Where(stu => stu.SAge < 26)
                                         .ToList();

5、聚合操作符 Min、Max、Sum、Count

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //聚合操作符
            var minAge = slist.Min(stu => stu.SAge);
            var maxAge = slist.Max(stu => stu.SAge);
            var sumAge = slist.Sum(stu => stu.SAge);
            var count = slist.Where(stu => stu.SAge < 26).Count();

6、排序

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //排序
            var query = slist.OrderBy(stu => stu.CId)
                            .ThenByDescending(stu => stu.SName)
                            .ThenBy(stu => stu.SAge);
                              

7、分区（分页）

        static void Main(string[] args)
        {
            List<Student> slist = new List<Student> { 
                new Student{ SId="S001",SName="彭飞",SAge=27,CId="C2" },
                new Student{ SId="S002",SName="张三",SAge=20,CId="C2" },
                new Student{ SId="S003",SName="李四",SAge=25,CId="C1" },
                new Student{ SId="S003",SName="王五",SAge=25,CId="C4" }
            };
            //分区
            var query = slist.Skip(1).Take(2);                              

谢谢。。。