【Linq】常用语法汇总

2024-07-01 11:23| 来源: 网络整理| 查看: 265

语言继承查询(Language Integrated Query, LINQ)，在C#编程语言中集成了查询语法，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，所以可以使用相同语法。

public class Book { public int Id { get; set; } /// /// 书名 /// public string BookName { get; set; } /// /// 作者id /// public int AutherId { get; set; } /// /// 类型 /// public string Type { get; set; } /// /// 售价 /// public decimal Price { get; set; } /// /// 销量 /// public int Sales { get; set; } } public class Auther { public int Id { get; set; } /// /// 作者 /// public string AutherName { get; set; } } public class Library { public string Address { get; set; } /// /// 书本 /// public List BookList { get; set; } }

1、条件查询（Where）

var query = from book in bookList where book.Price > 50 orderby book.Sales descending,book.BookName select book; //等同于 var query = bookList.Where(n => n.Price > 50).OrderByDescending(g => g.Sales).ThenBy(y => y.BookName);

　　需要注意的是，延迟查询

var ary = new List(){ "Aa", "Bb", "Cc"}; var a1 = ary.Where(n => n.Contains("a"));//["Aa"] ary.Add("Ga"); a1;//["Aa", "Ga"]

　　解决这个问题，只需要用 ToList();

var ary = new List(){ "Aa", "Bb", "Cc"}; var a1 = ary.Where(n => n.Contains("a")).ToList();//["Aa"] ary.Add("Ga"); a1;//["Aa"]

　　索引筛选

//销量大于50且为奇数行 var query = bookList.Where((n, index) => n.Sales > 50 && index%2 != 0);

2、复合查询（SelectMany）

//查找所有图书馆中的小说 var query = from library in libraryList from book in library.BookList where book.Type == "小说" select book; //等同于 query = libraryList.SelectMany(n => n.BookList).Where(g => g.Type == "小说"); //组织返回结果 query = libraryList.SelectMany(n => n.BookList, (n, g) => new {n.Address, g.BookName, g.Sales}).Where(y => y.Sales > 100);

3、集合连接

//内连接 var query1 = from book in bookList 　　　　　join auther in autherList on book.AutherId equals auther.Id 　　　　　select new { book.BookName, auther.AutherName }; //组连接 var query2 = from auther in autherList 　　　join book in bookList on auther.Id equals book.AutherId into items 　　　select new { auther.AutherName, Books = items }; //左外连接 var query3 = from book in bookList join auther in autherList on book.AutherId equals auther.Id into items from auther in items.DefaultIfEmpty() select new { book.BookName, AutherName = auther == default(Auther) ? "无" : auther.AutherName }; //多条件连接 var query4 = from book in bookList join auther in autherList on new {name = book.BookName, id = book.AutherId} equals new {name = auther.AutherName, id = auther.Id} select book;

4、排序（OrderBy、ThenBy）

var query = from book in bookList orderby book.Sales descending, book.AutherId, book.Price descending select book; //等同于 query = bookList.OrderByDescending(n => n.Sales).ThenBy(g => g.AutherId).ThenByDescending(y => y.Price);

5、分组（GroupBy）

//单一条件分组 var query = from book in bookList group book by book.Type into bs select bs.First(); //等同于 query = bookList.GroupBy(n => n.Type).Select(g => g.First()); //多条件分组 var query = from book in bookList group book by new { book.Type, book.AutherId } into bs select new { Type = bs.First().Type, AutherId = bs.First().AutherId, Count = bs.Count() }; //等同于 query = bookList.GroupBy(n => new {n.Type, n.AutherId}).Select(g => new { Type = g.First().Type, AutherId = g.First().AutherId, Count = g.Count() });

6、合并与分区（Zip、Take、Skip）

int[] numbers = { 1, 2, 3 }; string[] words = { "One", "Two", "Three", "Four" }; //元素依次组合，长度为较小的集合 IEnumerable zip = numbers.Zip(words, (n, g) => n + "=" + g);//["1=One", "2=Two", "3=Three"] //跳过集合的前n个元素 var skip = words.Skip(3);//["Four"] //获取集合的前n个元素,有延迟 var take = numbers.Take(2);//[1, 2] int pageSize = 3;//每页容量 int pageNum = 0;//页数 var page = words.Skip(pageSize * pageNum).Take(pageSize);//["One", "Two", "Three"]

7、集合操作（Distinct、Union、Concat、Intersect、Except）

int[] ary1 = {1, 2, 2, 4, 5}; int[] ary2 = {3, 5, 5, 6, 10, 7}; //合并,自动去重 var union = ary1.Union(ary2);//1, 2, 3, 4, 5, 6, 7, 10 //合并,不会去重 var concat = ary1.Concat(ary2);//1, 2, 2, 4, 5, 3, 5, 5, 6, 10, 7 //去重 var distict = ary1.Distinct();//1, 2, 4, 5 //取交集,自动去重 var intersect = ary1.Intersect(ary2);//5 //取补集,自动去重 var except = ary1.Except(ary2);//1, 2, 4

8、类型筛选（ofType）

object[] data = { "one", 1 , 2 ,"three"}; var query = data.ofType();

9、聚合操作符（Count、Sum、Min、Max、Average、Aggregate）

//个数 var count = bookList.Count(n => n.Sales > 50); //求和 var sum = bookList.Sum(n => n.Price); //最小值 var min = bookList.Min(n => n.Sales); //最大值 var max = bookList.Max(n => n.Price); //平均值 var average = bookList.Average(n => n.Sales); //累加,总销量 var aggregate1 = bookList.Select(n => n.Sales).Aggregate((g, y) => g + y); //累加，初始值 var aggregate2 = bookList.Select(n => n.Sales).Aggregate(10, (g, y) => g + y); //累加，初始值，结果处理 var aggregate3 = bookList.Select(n => n.Sales).Aggregate(10, (g, y) => g + y, result => result/100);

10、转换操作符（ToArray、ToDictionary、ToList、ToLookup、Cast）

Book[] ary = bookList.ToArray(); List list = bookList.ToList(); Dictionary dic = bookList.ToDictionary(n => n.Id); //转换成LookUp集合,key-以key分组的内部集合 ILookup look = bookList.ToLookup(n => n.Type); IEnumerable cast = ary.Cast();

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