Linq

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/

基本结构

static void ListFiles(string rootDir, string searchParttern)
{
    var ret = 
    from fileName in Directory.GetFiles(rootDir, searchParttern)
    let fi = new FileInfo(fileName)
    orderby fi.Length descending, fileName ascending
    select fi;
    
    // print data
}

where

var ret = from n in lst
where n >= 2
where n <= 4
select n

var ret = from n in lst
where n >= 2 && n <= 4 || n >= 8
select n

where还可以接受Lambda表达式做为过滤条件。一般将一些复杂的判断语句使用Lambda来表示。也可以使用Func定义一个委托来使用。

var ret = from person in personList
where(p=>p.Salary > 10000)
select person;

group by

有时需要将数据集合按某个key划分为多个集合

IEnumerable<IGrouping<int, Person>> ret =    // int为分组的key类型，Person为元素类型 
from person in personList
group person by person.DepartmentID

// 遍历
foreach(IGrouping<int, Person> group in ret)
{
    Console.WriteLine(string.Format("Department:{0}",group.Key));
    foreach(Person p in group)
    {
        Console.WriteLine(p.Name);
    }
}

into

使用into可以进行延续查询。into之前的变量失效，而新定义的变量生效。

 from person in personList
 where person.DepartmentID > 2
 select person
 into p
 where p.Salary > 10000
 select p;
 

使用from子句展开序列的序列

 var ret = from word in Keywords
 from character in word
 select character;
 
 
var numbers = new[]{1,2,3};
 var ret =
 from word in Keywords
 from n in numbers
 select new{word, n};
 

结果去重Distinct( )

 var ret =(from word in Keywords
 from character in word
 select character).Distinct( );
 

关联数据源join

 var ret =
 from person in personList
 join department in departmentList on person.DepartmentID equals person.DepartmentID
 select new {PersonName = p.Name, DepartmentName = d.DepartmentName};
 

序列类型过滤OfType

 var strRet = from s in mixArrayList.OfType()
 select s;
 

计数Count

 var ret = from str in Keywrods
 where str.Contains("L")
 select str;
 
 
Console.WriteLine(ret.Count());
 

---

例子：

查询一篇文章中含有指定单词的句子

 // Find sentences that contain all the terms in the wordsToMatch array.
 // Note that the number of terms to match is not specified at compile time.
 var sentenceQuery =
 from sentence in sentences
 let w = sentence.Split(new char[] { '.', '?', '!', ' ', ';', ':', ',' },StringSplitOptions.RemoveEmptyEntries)
 where w.Distinct().Intersect(wordsToMatch).Count() == wordsToMatch.Count()
 select sentence;