LINQ函数虽然和LINQ语句实现了同样的功能，但LINQ函数使用起来更加快捷。学过数据库的感觉LINQ语句都不难，但语句比较长。
会LINQ函数，才算会LINQ。
1、Where()，结果过滤
1 List<int> list = new List<int>() {1,2,3,4,5,6,7,8,9,10 }; 2 IEnumerable<int> newList = list.Where(a => a >= 3); 3 //查找大于3的数 4 //输出 5 foreach (var item in newList) 6 { 7 Console.WriteLine(item); 8 }结果：
345678910
2、Select()
Select()处理结果会传回一个对象，这个对象可以是现在对象 ，也可以是匿名类型。
在LINQ语句 中的select new语句 ，会自动被编译器转换为Select().
有如下代码：
1 List<int> list = new List<int>() {1,2,3,4,5,6,7,8,9,10 }; 2 var Num = from aa in list 3 where aa == 1 4 select new 5 { 6 Number = aa, 7 }; 8 Console.WriteLine(Num.FirstOrDefault().Number); 1 var SelectNum = list.Where(x => x == 1).Select(x => new 2 { 3 Number = x 4 });上面两段代码其实是一样的。
3、SelectMany()
SelectMany()类似于数据库中的CrossJoin
1 List<int> list1 = new List<int>() { 1, 2, 3, 4, 5, 6 }; 2 List<int> list2 = new List<int>() { 6, 4, 2, 7, 9, 0 }; 1 var query = list1.SelectMany(x => list2); 2 foreach (var item in query) 3 { 4 Console.WriteLine(item); 5 }运行结果就是将list2输出6次
下面来看看MSDN上面的代码
class PetOwner { public string Name { get; set; } public List<String> Pets { get; set; } } public static void SelectManyEx1() { PetOwner[] petOwners = { new PetOwner { Name="Higa, Sidney", Pets = new List<string>{ "Scruffy", "Sam" } }, new PetOwner { Name="Ashkenazi, Ronen", Pets = new List<string>{ "Walker", "Sugar" } }, new PetOwner { Name="Price, Vernette", Pets = new List<string>{ "Scratches", "Diesel" } } }; // Query using SelectMany(). IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets); Console.WriteLine("Using SelectMany():"); // Only one foreach loop is required to iterate // through the results since it is a // one-dimensional collection. foreach (string pet in query1) { Console.WriteLine(pet); } }运行结果：
Using SelectMany():ScruffySamWalkerSugarScratchesDiesel
这里其实很好理解 。就是1对2，输出2.
4、GroupBy()
GroupBy()会按照给定的key(keySelector)以及内容elementSelector，产生群组后的结果
GroupBy()设置了使用数列本身作为Key值，并且利用这个Key分组产生分组的数据。
1 List<int> list = new List<int>() { 1, 2, 3, 4, 5, 1, 5 }; 2 3 var query = list.GroupBy(x => x); 4 5 foreach (var item in query) 6 { 7 Console.WriteLine("NUMBER:{0} COUNT{1}",item.Key,item.Count()); 8 }运行结果：
NUMBER:1 COUNT2NUMBER:2 COUNT1NUMBER:3 COUNT1NUMBER:4 COUNT1NUMBER:5 COUNT2
1 foreach (var item in query) 2 { 3 for (int i = 0; i < item.Count(); i++) 4 { 5 Console.WriteLine(item.ElementAt(i)); 6 } 7 Console.WriteLine("======================="); 8 }运行结果：
11=======================2=======================3=======================4=======================55=======================
5、ToLookUp()
从 IEnumerable<T> 生成一个泛型 Lookup<TKey, TElement>。
ToLookUp()看起来和GroupBy()有些类似，但是它会另外生成一个新的集合对象，这个集合对象由ILookup<TKey,TElement>组成
1 var Marks = new[] 2 { 3 new {Mark = 90,Group = "A"}, 4 new {Mark = 80,Group = "B"}, 5 new {Mark = 70,Group = "C"}, 6 new {Mark = 60,Group = "D"} 7 }; 8 9 var lookUpValue = Marks.ToLookup(x => x.Group); 10 11 foreach (var item in lookUpValue) 12 { 13 Console.WriteLine("========Group:{0}=============",item.Key); 14 foreach (var result in item) 15 { 16 Console.WriteLine(result.Group + " " + result.Mark); 17 } 18 }运行结果
========Group:A=============A 90========Group:B=============B 80========Group:C=============C 70========Group:D=============D 60
另一段测试代码：
1 var MarksWithName = new[] 2 { 3 new {Mark = 90,Name="Jack",Group="A"}, 4 new {Mark = 80,Name="Jany",Group="B"}, 5 new {Mark = 70,Name = "Tom",Group="C"} 6 }; 7 8 var lookUpValue2 = MarksWithName.ToLookup(x => x.Group); 9 foreach (var item in lookUpValue2) 10 { 11 Console.WriteLine("============Group:{0}==============",item.Key); 12 foreach (var result in item) 13 { 14 Console.WriteLine("Name:{0} Mark:{1}",result.Name,result.Mark); 15 } 16 }运行结果：
============Group:A==============Name:Jack Mark:90============Group:B==============Name:Jany Mark:80============Group:C==============
6、Join
将两个集合进行联接，即数据库中的Inner Join
1 var Mark = new[] 2 { 3 new {Name = "C",Mark = 65}, 4 new {Name = "A",Mark = 70}, 5 new {Name = "B", Mark = 80} 6 7 }; 8 9 var Age = new[] 10 { 11 new {Name = "B",Age = 18}, 12 new {Name = "C",Age = 17}, 13 new {Name = "A",Age = 20} 14 }; //LINQ语句 var query = from mk in Mark join ae in Age on mk.Name equals ae.Name select new { Name = mk.Name, Mark = mk.Mark, Age = ae.Age }; //输出 foreach (var item in query) { Console.WriteLine("姓名{0} 成绩{1} 年龄{2}",item.Name,item.Mark,item.Age); } 1 //LINQ函数 2 3 var query2 = Mark.Join(Age, mk => mk.Name, ae => ae.Name, (mk, ae) => new { Name = mk.Name,Mark = mk.Mark,Age = ae.Age }); 4 5 foreach (var item in query2) 6 { 7 Console.WriteLine("姓名{0} 成绩{1} 年龄{2}", item.Name, item.Mark, item.Age); 8 }上面两段代码输出的结果都是：
姓名C 成绩65 年龄17姓名A 成绩70 年龄20姓名B 成绩80 年龄18
7、GroupJoin
将两个集合进行联接，并对结果进行分组。下面的例子就是以国家名字为组，显示其对应的省份信息。
1 class Country 2 { 3 public string CountryName { get; set; } 4 } 5 6 class Province 7 { 8 public Country OwnerCountry { get; set; } 9 public string ProvinceName { get; set; } 10 public string ProvinceSize { get; set; } 11 public string ProvinceSpot { get; set; } 12 } 1 Country c1 = new Country() 2 { 3 CountryName = "China" 4 }; 5 6 Country c2 = new Country() 7 { 8 CountryName = "America" 9 }; 10 11 Country c3 = new Country() 12 { 13 CountryName = "Korea" 14 }; 15 16 Country c4 = new Country() 17 { 18 CountryName = "Russian" 19 }; 20 21 22 Province p1 = new Province() 23 { 24 ProvinceName = "Hunan", 25 ProvinceSize = "200", 26 ProvinceSpot = "Old City", 27 OwnerCountry = c1 28 }; 29 30 Province p2 = new Province() 31 { 32 ProvinceName = "Chicago", 33 ProvinceSize = "150", 34 ProvinceSpot = "Linkin Park", 35 OwnerCountry = c2 36 }; 37 38 Province p3 = new Province() 39 { 40 ProvinceName = "Seoul", 41 ProvinceSize = "100", 42 ProvinceSpot = "Tian Tian World", 43 OwnerCountry = c3, 44 }; 45 46 Province p4 = new Province() 47 { 48 ProvinceName = "Hunan", 49 ProvinceSize = "200", 50 ProvinceSpot = "Tian Meng Mountain", 51 OwnerCountry = c1 52 }; 53 54 Province p5 = new Province() 55 { 56 ProvinceName = "Moscow", 57 ProvinceSize = "80", 58 ProvinceSpot = "Moscow University", 59 OwnerCountry = c4 60 }; 61 62 Province p6 = new Province() 63 { 64 ProvinceName = "Moscow", 65 ProvinceSize = "80", 66 ProvinceSpot = "HelloWorld", 67 OwnerCountry = c4 68 };声明4个Country类和6个Province类，并放入countries和provinces集合
1 List<Province> provinces = new List<Province>() 2 { 3 p1,p2,p3,p4,p5,p6 4 }; 5 6 List<Country> countries = new List<Country>() 7 { 8 c1,c2,c3,c4 9 };先来看看Join的结果
1 var joinResult = countries.Join(provinces, 2 country => country, 3 province => province.OwnerCountry, 4 (country, province) => 5 new 6 { 7 CountryName = country.CountryName, 8 ProvinceName = province.ProvinceName, 9 Size = province.ProvinceSize, 10 Spot = province.ProvinceSpot, 11 }); 12 Console.WriteLine("--------------------------------"); 13 foreach (var item in joinResult) 14 { 15 Console.WriteLine(item.CountryName + " " + item.ProvinceName + " " + item.Size + " " + item.Spot); 16 }运行结果：
--------------------------------China Hunan 200 Old CityChina Hunan 200 Tian Meng MountainAmerica Chicago 150 Linkin ParkKorea Seoul 100 Tian Tian WorldRussian Moscow 80 Moscow UniversityRussian Moscow 80 HelloWorld--------------------------------
再看看GroupJoin的结果
1 var groupJoinResult = countries.GroupJoin(provinces, 2 country=>country, 3 province=>province.OwnerCountry, 4 (country,province)=> 5 new 6 { 7 Name = country.CountryName, 8 ProvinceSet = province.Select(x=>x) 9 }); 10 foreach (var item in groupJoinResult) 11 { 12 Console.WriteLine("******************"); 13 Console.WriteLine(item.Name); 14 foreach (var p in item.ProvinceSet) 15 { 16 Console.WriteLine(p.ProvinceName + " " + p.ProvinceSize + " " + p.ProvinceSpot); 17 } 18 }运行结果：
******************ChinaHunan 200 Old CityHunan 200 Tian Meng Mountain******************AmericaChicago 150 Linkin Park******************KoreaSeoul 100 Tian Tian World******************RussianMoscow 80 Moscow UniversityMoscow 80 HelloWorld
8、Distinct
去除集合中的重复结果
值类型
1 class Program 2 { 3 static void Main(string[] args) 4 { 5 List<int> valueList = new List<int>() { 1, 1, 2, 2, 3, 3, 4, 4, }; 6 var distinctValueList = valueList.Distinct().ToList(); 7 distinctValueList.ForEach(x => Console.WriteLine(x)); 8 } 9 }运行结果
1234
引用类型
1 public class PrintImage 2 { 3 public int ImageKey { get; set; } 4 5 public string ImagePath { get; set; } 6 } 7 8 //定义方法以支持对象的相等比较。 9 //https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.generic.iequalitycomparer-1?redirectedfrom=MSDN&view=netframework-4.8 10 class PrintImageEqualityComparer : IEqualityComparer<PrintImage> 11 { 12 public bool Equals(PrintImage p1, PrintImage p2) 13 { 14 if (p1 == null && p2 == null) 15 return true; 16 else if (p1 == null || p2 == null) 17 return false; 18 else if (p1.ImageKey == p2.ImageKey) 19 return true; 20 else 21 return false; 22 } 23 24 public int GetHashCode(PrintImage pImage) 25 { 26 return pImage.ImageKey.GetHashCode(); 27 } 28 } 29 30 class Program 31 { 32 static void Main(string[] args) 33 { 34 List<PrintImage> objList = new List<PrintImage>() 35 { 36 new PrintImage() 37 { 38 ImageKey = 1, 39 ImagePath = "D:\\crawler\\image1" 40 }, 41 new PrintImage() 42 { 43 ImageKey = 1, 44 ImagePath = "D:\\crawler\\image2" 45 }, 46 new PrintImage() 47 { 48 ImageKey = 2, 49 ImagePath = "D:\\crawler\\image3" 50 }, 51 new PrintImage() 52 { 53 ImageKey = 3, 54 ImagePath = "D:\\crawler\\image4" 55 } 56 }; 57 58 //使用实现了IEqualityComparer<T>的PrintImageEqualityComparer类对PrintImage对象进行比较 59 //通过使用Distincet去除了集合中的第二个对象 60 var distinctObjList = objList.Distinct(new PrintImageEqualityComparer()).ToList(); 61 distinctObjList.ForEach(x => { Console.WriteLine(x.ImageKey); Console.WriteLine(x.ImagePath); }); 62 } 63 }运行结果：
1D:\crawler\image12D:\crawler\image33D:\crawler\image4
转载于:https://www.cnblogs.com/zhaotianff/p/6236062.html
相关资源：nathaliebarruel_4_26012021：Projet n°4开放式教室-源码