【Linq】常用语法汇总

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

1、条件查询（Where）

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

　　需要注意的是，延迟查询

var ary = new List<string>(){ "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<string>(){ "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 >  && index% != );

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 > );

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 = { , ,  };
string[] words = { "One", "Two", "Three", "Four" };
//元素依次组合，长度为较小的集合
IEnumerable<string> 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();//["Four"]
//获取集合的前n个元素,有延迟
var take = numbers.Take();//[1, 2]

int pageSize = ;//每页容量
int pageNum = ;//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 = {, , , , };
int[] ary2 = {, , , , , };

//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",  ,  ,"three"};
var query = data.ofType<string>();

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

//个数
var count = bookList.Count(n => n.Sales > );
//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(, (g, y) => g + y);
//累加，初始值，结果处理
var aggregate3 = bookList.Select(n => n.Sales).Aggregate(, (g, y) => g + y, result => result/);

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

Book[] ary = bookList.ToArray();

List<Book> list = bookList.ToList();

Dictionary<int, Book> dic = bookList.ToDictionary<Book, int>(n => n.Id);

//转换成LookUp集合,key-以key分组的内部集合
ILookup<string, Book> look = bookList.ToLookup(n => n.Type);

IEnumerable<Book> cast = ary.Cast<Book>();