C#Linq查询操作使用举例
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2022-05-12 17:25:10
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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 |
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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