.NET框架-LINQ 9类操作符使用举例

Linq标准查询操作符

语言集查询 Ganmge hteg.ratedQuw,LINQ,集 成了C#编程语言中的查询语法 ,可 以用相同的语法访问不同的数据源。 LINQ提 供了不同数据源的抽象层 ,所 以可以使用相同的语法。

1筛选操作符

筛选操作符定义了返回元素的条件。

筛选操作符 描述 where 使用谓词，返回布尔值 OfType< TResult> 根据类型筛选元素

应用举例：
where用法：

 var racers = from r in Formulal.Racers                         where r.Wins > 15 && r.Country == "Brazil" select r;

OfType< T>用法：

object[] data = {"one", 2, "three", 4, 5, "six"};            var rtnData = data.OfType<string>(); //返回类型为string的数据元素集合

2投射操作符

把对象转化为另一个类型的新对象

投射操作符 描述 select selectMany

用法举例：
select用法

var racers = from r in Formulal.Racers                         where r.Wins > 15 && r.Country == "Brazil"                         select new                         {                             Name = string.Format("{0} {1}", r.FirstName, r.LastName),                             Country = r.Country,                             Wins = r.Wins                         };   //输出为含有Name,Country,Wins共计3个属性的对象集合

selectMany用法

            var ferrariDrivers = Formulal.Racers.            SelectMany(r => r.Cars,            (r, c) => new { Racer = r, Car = c }).            Where(r => r.Car == "Ferrari").            OrderBy(r => r.Racer.LastName).            Select(r => r.Racer.FirstName + " " + r.Racer.LastName);//筛选出驾驶法拉利的选手集合

3排序操作符

改变返回元素的顺序

排序操作符 描述 OrderBy 升序排序 OrderByDescending 降序排序 ThenBy 第一次排序结果类似，用第二次升序排序 ThenByDescending 第一次排序结果类似，用第二次降序排序 Reverse 反转集合中元素的顺序

举例：

    var racers = from r in Formulal.Racers                          where r.Country == "Brazil"                          orderby r.Wins descending                          select new                          {                              Name = string.Format("{0} {1}", r.FirstName, r.LastName),                              Country = r.Country,                              Wins = r.Wins                          };//国家为巴西，按照胜场次数降序排列

4连接运算符

用于合并不直接相关的集合

连接运算符 描述 Join 可以根据键选择器函数连接连接2个集合 GroupJoin 连接两个集合,组合其结果

//Join连接            //获取冠军年份在2003年后的选手            var racer5 = from r in Formulal.Racers                from y in r.Years                where y > 2003                select new                {                    Year = y,                    Name = string.Format("{0} {1}", r.FirstName, r.LastName)                };            //获取冠军年份在2003年后的团队            var team1 = from r in Formulal.ChampionTeams                         from y in r.Years                         where y > 2003                         select new                         {                             Year = y,                             Name = string.Format("{0}", r.Name)                         };            //连接            var racerAndTeams = from r in racer5                join t in team1 on r.Year equals t.Year                select new                {                    Yearj= r.Year,                    Racerj = r.Name,                    Teamj = t.Name                };

结果：

Yearj Racerj Teamj Yearj=2004 Racerj=Michael Schumacher Teamj =Ferrari Yearj=2005 Racerj=Fernando Alonso Teamj =Renault Yearj=2006 Racerj=Fernando Alonso Teamj =Renault Yearj=2007 Racerj=Kimi Räikkönen Teamj =Ferrari Yearj=2008 Racerj=Lewis Hamilton Teamj =Ferrari Yearj=2014 Racerj=Lewis Hamilton Teamj =Mercedes Yearj=2015 Racerj=Lewis Hamilton Teamj =Mercedes Yearj=2009 Racerj=Jenson Button Teamj =Brawn GP Yearj=2010 Racerj=Sebastian Vettel Teamj =Red Bull Racing Yearj=2011 Racerj=Sebastian Vettel Teamj =Red Bull Racing Yearj=2012 Racerj=Sebastian Vettel Teamj =Red Bull Racing Yearj=2013 Racerj=Sebastian Vettel Teamj =Red Bull Racing

5组合运算符

把数据放在组中

组合运算符 描述 GroupBy 组合有公共键的元素 ToLookUp 通过创建一个一对多的字典，来组合元素。

GroupBy用法举例：

 var countries = from r in Formulal.Racers                            group r by r.Country into g                            orderby g.Count() descending, g.Key                            where g.Count() > 2                            select new                             {                                 Country = g.Key,                                 Count = g.Count()                             };//获取冠军次数大于2的国家

结果：

Country Count UK 10 Brazil 3 Finland 3

如果分组需要根据多个字段，则应该这样写：

            //按组合键分组后，每组元素个数大于2的分组，按降序排序            var rtnByVal1 = from item in dataSource                            group item by new { item.Val1, item.Val2 }                                into g                                where g.Count()>1                                orderby g.Count() descending                                select g;            //按Val1和Val2组合为字典键，元素个数为值            var dict = rtnByVal1.ToDictionary(g=>g.First(),g=>g.Count());

6量词操作符

如果元素序列满足指定的条件 ,量词操作符就返回布尔值

组合运算符 描述 Any 确定集合中是否有满足谓词函数的元素 All 确定集合中是否都满足谓词函数的元素 Contains 检查某个元素是否存在集合中

            List<int> intList1 = new List<int>(5) {1, 2, 3, 4, 5};                       bool any3 = intList1.Any(r => r == 3);//确定集合中是否有3，返回true            bool any0 = intList1.Any(r => r == 0);//确定集合中是否有0，返回false            bool allMoreZero = intList1.All(r => r > 0);//确定序列中所有元素都满足大于0，返回true            bool allMore2 = intList1.All(r => r > 2); //返回false            bool contains3 = intList1.Contains(3); //元素3是否在集合中，true

7分区操作符

分区操作符，返回一个子集。使用它们可以得到部分结果。

分区运算符 描述 Take 必须制定提取元素的个数 Skip 跳过指定的元素个数，提取其他元素 TakeWhile 提取条件为真的元素 SkipWhile 跳过条件为真的元素

            int pageSize = 5;            int numberPages = (int)Math.Ceiling(Formulal.Racers.Count() / (double)pageSize);            for (int page = 0; page < numberPages; page++)            {                var racersPartition =                    (from r in Formulal.Racers                     orderby r.LastName, r.FirstName                     select r.FirstName + " " + r.LastName).Skip(page * pageSize).Take(pageSize);                                   }

结果：

Page 0Fernando AlonsMario AndrettiAlberto AscariJack BrabhamJenson ButtonPage 1Jim ClarkJuan Manuel FaNino FarinaEmerson FittipaldiMika HakkinenPage 2Lewis HamiltonMike HawthornDamon HillGraham HillPhil Hill

8集合操作符（Set operators）

集合操作符返回 一 个集合。除了Distinct之外，其他都需要2个集合。

集合操作符 描述 Distinct 从集合中删除重复的元素 Union 返回出现在其中一个集合中的唯一元素 Intersect 交集 Except 只出现在一个集合中的元素 Zip 通过使用指定的谓词函数合并2个集合

应用举例：

            List<int> intList1 = new List<int>(5) {1, 2, 3, 4, 5,3};            List<int> intList2 = new List<int>(3) {2, 5, 0};            var delDuplic = intList1.Distinct(); //去重，结果{1,2,3,4,5}            var unionList = intList1.Union(intList2); //并集,结果{1,2,3,4,5,0}            var interList = intList1.Intersect(intList2); //交集，结果{2,5}            var exceptList = intList1.Except(intList2);//差集，结果{1,3,4}            var exceptList2 = intList2.Except(intList1);//差集，结果{0}            var unionList2 = intList1.Zip(intList2,((i1, i2) => i1+", "+i2)); 结果：{{1,2}{2,5}{3,0}}

9元素操作符（Element operators）

这些元素操作符仅返回一 个元素。

元素操作符 描述 First 返回第一个满足条件的元素 FirstOrDefault 返回第一个满足条件的元素，但如果没有找到满足条件的元素,就返回类型的默认值 Last 返回最后一个满足条件的元素 LastOrDefault 返回最后一个满足条件的元素，但如果没有找到满足条件的元素,就返回类型的默认值 ElementAt 指定了要返回的元素的位置 ElementAtOrDefault 指定了要返回的元素的位置，但如果没有找到满足条件的元素,就返回类型的默认值 Single 只返回一个满足条件的元素，如果有多个满足条件，就抛出一个异常。 SingleOrDefault 只返回一个满足条件的元素，但如果没有找到满足条件的元素,就返回类型的默认值

应用举例：

List<int> intList1 = new List<int>(5) { 1, 8, 3, 4, 5, 3 };int firstMore1 = intList1.First(r => r > 1);//结果：8int firstMore17 = intList1.FirstOrDefault(r => r > 17); //结果：0（因为不存在）int lastMore3 = intList1.Last(r => r > 3); //结果5int last = intList1.ElementAt(2); //结果3,（索引从0开始）

10聚合操作符（Aggregate operators）

聚合操作符计算集合的一个值。利用这些聚合操作符 ,可 以计算所有值的总和、所有元
素的个数、值最大和最小的元素 ,以 及平均值等

聚合操作符 描述 Count 所有值的个数 Sum 所有值的综合 Min 所有值的的最小值 Max 所有值的的最大值 Average 所有值的平均数

            public class MyIntClass            {                public string Name { get; set; }                public int Integar { get; set; }                public MyIntClass(string name, int i)                {                    this.Name = name;                    this.Integar = Integar;                }            }            List<MyIntClass> intList1 = new List<MyIntClass>(5)             {                 new MyIntClass("first",1),                new MyIntClass("second",8) ,                new MyIntClass("third",3) ,                new MyIntClass("fourth",4) ,                new MyIntClass("fifth",5) ,                new MyIntClass("sixth",3)             };            int count = intList1.Count;            int sum = intList1.Sum(r=>r.Integar);            int min = intList1.Min(r => r.Integar);            int max = intList1.Max(r => r.Integar);            double average = intList1.Average(r => r.Integar);

11转换操作符（Conversion operators）

转换操作符将集合转换为数组 :IEnumberable、 IList, IDictionary 等。

Conversion operators 描述 ToArray 集合转化为Array AsEnumerable 返回类型为IEnumerable< T> ToList 集合转化为List ToDictionary 集合转化为Dictionary Cast< TResult> 映射

还是上面的例子

         IEnumerable<MyIntClass> ienuList = from r in intList1 where r.Integar > 3 select r; //返回默认的IEnumerable集合         List<MyIntClass> ienuList2 = (from r in intList1 where r.Integar > 3 select r).ToList(); //返回List         MyIntClass[] ienuList2 = (from r in intList1 where r.Integar > 3 select r).ToArray();//返回数组         var dict = (from r in intList1 where r.Integar > 3 select r).ToDictionary(r=>r.Name,r=>r.Integar); //字典，key是name, value：Integar         IEnumerable<MyIntClass> ienuList2 = (from r in intList1 where r.Integar > 3 select r).AsEnumerable(); 

12生成操作符（Generation operators）

这些生成操作符返回 一 个新集合

Generation operators 描述 Empty 集合是空的 Range 返回一系列数字 Repeat 返回始终重复一个值的集合

IEnumerable<int> ints = Enumerable.Range(3, 10);//{3,4,5,6,7,8,9,10,11,12}IEnumerable<int> emptyInts =  Enumerable.Empty<int>();//生成一个空集合IEnumerable<int> ints2= Enumerable.Repeat(6,8);//生成8个6的集合

附：展示所用到的实体类和数据

选手实体类

   //选手实体类   public class Racer     {        public Racer(string firstName = null, string lastName = null, string country = null, int starts = 0,int wins = 0, IEnumerable<int> years = null, IEnumerable<string> cars = null)        {            this.FirstName = firstName;            this.LastName = lastName;            this.Country = country;            this.Starts = starts;            this.Wins = wins;            var yearList = new List<int>();            if (years != null)            {                foreach (var year in years)                {                    yearList.Add(year);                }                this.Years = yearList.ToArray();            }            if (cars != null)            {                var carList = new List<string>();                foreach (var car in cars)                {                    carList.Add(car);                }                this.Cars = carList.ToArray();            }           public string FirstName { get; set; }           public string LastName { get; set; }           //赢得比赛的次数           public int Wins { get; set; }           //所属国家           public string Country { get; set; }           //开始做的年龄           public int Starts { get; set; }           //车型数组           public string[] Cars { get; private set; }           //赢得冠军的年份           public int[] Years { get; private set; }            }      }

选手数据

//选手List        public static List<Racer> Racers = new List<Racer>(40)        {                         new Racer("Nino", "Farina", "Italy", 33, 5, new int[] { 1950 }, new string[] { "Alfa Romeo" }),                new Racer("Alberto", "Ascari", "Italy", 32, 10, new int[] { 1952, 1953 }, new string[] { "Ferrari" }),                new Racer("Juan Manuel", "Fangio", "Argentina", 51, 24, new int[] { 1951, 1954, 1955, 1956, 1957 }, new string[] { "Alfa Romeo", "Maserati", "Mercedes", "Ferrari" }),                new Racer("Mike", "Hawthorn", "UK", 45, 3, new int[] { 1958 }, new string[] { "Ferrari" }),                new Racer("Phil", "Hill", "USA", 48, 3, new int[] { 1961 }, new string[] { "Ferrari" }),                new Racer("John", "Surtees", "UK", 111, 6, new int[] { 1964 }, new string[] { "Ferrari" }),                new Racer("Jim", "Clark", "UK", 72, 25, new int[] { 1963, 1965 }, new string[] { "Lotus" }),                new Racer("Jack", "Brabham", "Australia", 125, 14, new int[] { 1959, 1960, 1966 }, new string[] { "Cooper", "Brabham" }),                new Racer("Denny", "Hulme", "New Zealand", 112, 8, new int[] { 1967 }, new string[] { "Brabham" }),                new Racer("Graham", "Hill", "UK", 176, 14, new int[] { 1962, 1968 }, new string[] { "BRM", "Lotus" }),                new Racer("Jochen", "Rindt", "Austria", 60, 6, new int[] { 1970 }, new string[] { "Lotus" }),                new Racer("Jackie", "Stewart", "UK", 99, 27, new int[] { 1969, 1971, 1973 }, new string[] { "Matra", "Tyrrell" }),                new Racer("Emerson", "Fittipaldi", "Brazil", 143, 14, new int[] { 1972, 1974 }, new string[] { "Lotus", "McLaren" }),                new Racer("James", "Hunt", "UK", 91, 10, new int[] { 1976 }, new string[] { "McLaren" }),                new Racer("Mario", "Andretti", "USA", 128, 12, new int[] { 1978 }, new string[] { "Lotus" }),                new Racer("Jody", "Scheckter", "South Africa", 112, 10, new int[] { 1979 }, new string[] { "Ferrari" }),                new Racer("Alan", "Jones", "Australia", 115, 12, new int[] { 1980 }, new string[] { "Williams" }),                new Racer("Keke", "Rosberg", "Finland", 114, 5, new int[] { 1982 }, new string[] { "Williams" }),                new Racer("Niki", "Lauda", "Austria", 173, 25, new int[] { 1975, 1977, 1984 }, new string[] { "Ferrari", "McLaren" }),                new Racer("Nelson", "Piquet", "Brazil", 204, 23, new int[] { 1981, 1983, 1987 }, new string[] { "Brabham", "Williams" }),                new Racer("Ayrton", "Senna", "Brazil", 161, 41, new int[] { 1988, 1990, 1991 }, new string[] { "McLaren" }),                new Racer("Nigel", "Mansell", "UK", 187, 31, new int[] { 1992 }, new string[] { "Williams" }),                new Racer("Alain", "Prost", "France", 197, 51, new int[] { 1985, 1986, 1989, 1993 }, new string[] { "McLaren", "Williams" }),                new Racer("Damon", "Hill", "UK", 114, 22, new int[] { 1996 }, new string[] { "Williams" }),                new Racer("Jacques", "Villeneuve", "Canada", 165, 11, new int[] { 1997 }, new string[] { "Williams" }),                new Racer("Mika", "Hakkinen", "Finland", 160, 20, new int[] { 1998, 1999 }, new string[] { "McLaren" }),                new Racer("Michael", "Schumacher", "Germany", 287, 91, new int[] { 1994, 1995, 2000, 2001, 2002, 2003, 2004 }, new string[] { "Benetton", "Ferrari" }),                new Racer("Fernando", "Alonso", "Spain", 252, 32, new int[] { 2005, 2006 }, new string[] { "Renault" }),                new Racer("Kimi", "Räikkönen", "Finland", 230, 20, new int[] { 2007 }, new string[] { "Ferrari" }),                new Racer("Lewis", "Hamilton", "UK", 166, 43, new int[] { 2008, 2014, 2015 }, new string[] { "McLaren", "Mercedes" }),                new Racer("Jenson", "Button", "UK", 283, 15, new int[] { 2009 }, new string[] { "Brawn GP" }),                new Racer("Sebastian", "Vettel", "Germany", 156, 42, new int[] { 2010, 2011, 2012, 2013 }, new string[] { "Red Bull Racing" })        };

团队实体类

    [Serializable]    public class Team    {        public Team(string name, params int[] years)        {            this.Name = name;            this.Years = years;        }        public string Name { get; private set; }        public int[] Years { get; private set; }    }

团队数据

        //冠军团队List        public static List<Team> ChampionTeams = new List<Team>()        {            new Team("Vanwall", 1958),            new Team("Cooper", 1959, 1960),            new Team("Ferrari", 1961, 1964, 1975, 1976, 1977, 1979, 1982, 1983, 1999, 2000, 2001, 2002, 2003, 2004, 2007,                2008),            new Team("BRM", 1962),            new Team("Lotus", 1963, 1965, 1968, 1970, 1972, 1973, 1978),            new Team("Brabham", 1966, 1967),            new Team("Matra", 1969),            new Team("Tyrrell", 1971),            new Team("McLaren", 1974, 1984, 1985, 1988, 1989, 1990, 1991, 1998),            new Team("Williams", 1980, 1981, 1986, 1987, 1992, 1993, 1994, 1996, 1997),            new Team("Benetton", 1995),            new Team("Renault", 2005, 2006),            new Team("Brawn GP", 2009),            new Team("Red Bull Racing", 2010, 2011, 2012, 2013),            new Team("Mercedes", 2014, 2015)        };