C#.net工作笔记001---Linq对象查询,排序,分组,去重在工作中的使用

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1.对list中的某两个字段按照升序排序

testlist=testlist.OrderBy(s=>new{s.cd1,s.cd2}).ToList<TestDto>();

List<Student> stu = (List<Student>)Session["StudentList"];

下面是详细一点的排序:

Linq表达式:

//按学号降序

List<Student> stuList = (from s instu orderby s.stuNOdescending select s).ToList<Student>();

//按学号升序

List<Student> stuList = (from s instu orderby s.stuNO select s).ToList<Student>();

使用Lambda表达式排序: //按学号降序
单字段：List<Student> stuList= stu.OrderByDescending(s=> s.orderid).ToList<Student>();
多字段：List<Student> stuList= stu.OrderByDescending(s=> new{s.stuNO,s.stuName}).ToList<Student>();

//按学号升序
单字段：List<Student> stuList= stu.OrderBy(s=> s.stuNO).ToList<Student>();
多字段：List<Student> stuList= stu.OrderBy(s=> new{s.stuNO,s.stuName}).ToList<Student>();

多字段主次顺序排序情况，先按no排序，再按name排序
List<Student> stuList= stu.OrderBy(s=> s.stuNO).ThenBy(s=> s.stuName).ToList<Student>();

List<Student> stuList= stu.OrderBy(s=> new{ s.stuNO }).ThenBy(s=> new{s.stuName}).ToList<Student>();

//2.先排序再分组

testlist =(List<TestDto>)(from m in testlist orderby m.orderno group m by new {m.nroderno , m.ordernodtl} into mygroup select mygroup);

//3.获取一个list中的某个字段最大的值

（from c in biaoming where c.名称== 条件 select c.ManualID).Max();

//4.按照条件查询,先查询再排序

LstTmp = (from m in TmpLst where (m.testcd == "1") && (m.testcd2 == "2") select m).ToList<TestDto>().OrderBy(s => new { s.sor1, s.sort2 }).ToList<TestDto>();

//5.LINQ中如何按实体的某个属性去重

问题描述

比如有如下实体集合：

Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"

如何使用LINQ按 Person.Id 去重，返回的集合只包含 Person1 和 Person3 ？

方案一

创建一个静态扩展类：这个主意一下,这个可以放到一个命名空间下,然后再写一个类:

像下面这样,这样才能用:

static class ListExtendMethod{public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector){HashSet<TKey> seenKeys = new HashSet<TKey>();foreach (TSource element in source){if (seenKeys.Add(keySelector(element))){yield return element;}}}}

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
{HashSet<TKey> seenKeys = new HashSet<TKey>();foreach (TSource element in source){if (seenKeys.Add(keySelector(element))){yield return element;}}
}

调用方法：

var query = people.DistinctBy(p => p.Id);

如果需要按多个属性去重，则可以使用匿名对象，如：

var query = people.DistinctBy(p => new { p.Id, p.Name });

方案二

List<Person> distinctPeople = allPeople.GroupBy(p => p.PersonId).Select(g => g.First()).ToList();

多属性去重：

List<Person> distinctPeople = allPeople.GroupBy(p => new {p.PersonId, p.FavoriteColor} ).Select(g => g.First()).ToList();

方案三

var uniquePeople = from p in peoplegroup p by new {p.ID} //or group by new {p.ID, p.Name, p.Whatever}into mygroupselect mygroup.FirstOrDefault();

方案四

Persons.ToLookup(p => p.Id).Select(coll => coll.First());

方案五

var result = people.Where(p => !people.Any(q => (p != q && p.Id == q.Id)));