C#Linq查询操作使用举例

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
OfType3bb63614c352f1576fbc58977cc251c2	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;

OfTypeb32f285eaba7a6752dff0bc229700674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

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 Alons
Mario Andretti
Alberto Ascari
Jack Brabham
Jenson ButtonPage 1Jim Clark
Juan Manuel Fa
Nino Farina
Emerson Fittipaldi
Mika Hakkinen
Page 2Lewis Hamilton
Mike Hawthorn
Damon Hill
Graham Hill
Phil 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IEnumerableb32f285eaba7a6752dff0bc229700674
ToList	集合转化为List
ToDictionary	集合转化为Dictionary
Cast3bb63614c352f1576fbc58977cc251c2	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; }   
         }
      }
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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)
        };

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