C#.Net Tutorial

C#Linq query operation usage examples

黄舟

Mar 04, 2017 am 10:48 AM

Linq standard query operator

Language set query Ganmge hteg.ratedQuw, LINQ, integrates queries in the C# programming language Syntax, you can use the same syntax to access different data sources. LINQ provides an abstraction layer for different data sources, so the same syntax can be used.

1Filter operator

Filter operator defines the conditions for returning elements.

##where Use predicates to return a Boolean valueOfTypeFilter elements based on type

Filter Operator	Description

Application examples:

where Usage:

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

OfTypeUsage:

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 Projection operator

Convert the object into a new object of another type

Projection OperatorDescription##selectselectMany##Usage example: select usage

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 usage

            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 Sorting operator

Change the order of returned elements

Sort operatorDescription OrderByAscending orderOrderByDescendingDescending orderThenByThe first sorting result is similar, use the second ascending orderThenByDescending The result of the first sort is similar. Use the second descending sort Reverse to reverse the order of the elements in the setExample:

    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 Connection operator

is used to merge sets that are not directly related

Connection operatorDescription##JoinCan connect 2 collections according to the key selector functionJoin two collections and combine the results##

//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
                };


	GroupJoin
	Result:

Yearj

RacerjTeamjRacerj=Michael SchumacherTeamj =FerrariTeamj =RenaultTeamj =Renault##Yearj=2007Racerj=Kimi RäikkönenTeamj =FerrariYearj=2008Racerj=Lewis HamiltonTeamj =FerrariYearj=2014Racerj=Lewis HamiltonTeamj =Mercedes5Combining OperatorsPut data in groups

		##Yearj=2004
	Yearj=2005	Racerj=Fernando Alonso
	Yearj=2006	Racerj=Fernando Alonso



##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

Combining Operators

Description

ToLookUpGroupBy usage example: Result: ##Country

GroupBy	Group elements with a common key
By creating a pair Multiple dictionaries to combine elements.
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

GroupBy

Group elements with a common key

By creating a pair Multiple dictionaries to combine elements.

 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

Count

UK10##Brazil336 Quantifier operatorIf the element sequence satisfies the specified condition, The quantifier operator returns a Boolean valueCombining operator


	Finland

Description

AnyAllDetermine whether all the elements in the set satisfy the predicate functionCheck whether an element exists in the collection7 Partition operatorPartition Operator, returns a subset. Use them to get partial results. Partition Operator

Determine whether there are elements in the set that satisfy the predicate function

Contains

            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

Description

TakeSkipSkip the specified number of elements and extract other elementsExtract elements whose condition is trueSkip elements whose condition is true

The number of elements to be extracted must be specified

TakeWhile

SkipWhile

            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IEnumerable
ToList	集合转化为List
ToDictionary	集合转化为Dictionary
Cast	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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