1、linq中交集、并集、差集的用法
简单的交集、并集、差集用法如下：
List<string> ListA = new List<string>();
List<string> ListB = new List<string>();
List<string> ListResult = new List<string>();
ListResult = ListA.Distinct().ToList();//去重
ListResult = ListA.Except(ListB).ToList();//差集
ListResult = ListA.Union(ListB).ToList(); //并集
ListResult = ListA.Intersect(ListB).ToList();//交集
根据某个字段分组获取每个组别的一条数据
var uniquePeople = from p in hpList
group p by new { p.SoleKey} //or group by new {p.ID, p.Name, p.Whatever}
into mygroup
select mygroup.FirstOrDefault();
若上面的例子不是List类型，而是List，则需要对XXXModel进行处理。
步骤如下：
(1)先定义Model。
public class ItemModel
{
public string ItemCode { get; set; }
public string ItemName { get; set; }
}
(2)定义如何Model间如何比较。若不定义，比较的是两个引用。
public class ItemModelComparer : IEqualityComparer<ItemModel>
{
//比较
public bool Equals(ItemModel x, ItemModel y)
{
bool checkFlag = true;
if (Object.ReferenceEquals(x, y))
{
checkFlag = true;
}
else if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
{
checkFlag = false;
}
else
{
if (x.ItemCode == y.ItemCode) //若Model有多个条件则需要添加。例如 if(x.ItemCode==y.ItemCode && x.Other==y.Other)
{
checkFlag = true;
}
else
{
checkFlag = false;
}
}
return checkFlag;
}
//实现获取哈希值
public int GetHashCode(ItemModel model)
{
if (Object.ReferenceEquals(model, null)) return 0;
int hashNurse = model.ItemCode.GetHashCode();
// 若有多个条件，则哈希值也需要进行计算
// int hashOther=model.Other.GetHashCode();
// int resultHash=hashNurse^hashOther;
// return resultHash;
return hashNurse;
}
}
(3)具体使用如下
List<ItemModel> ListA = new List<ItemModel>();
List<ItemModel> ListB = new List<ItemModel>();
List<ItemModel> ListResult = new List<ItemModel>();
ListResult = ListA.Distinct(new ItemModelComparer()).ToList();//去重
ListResult = ListA.Except(ListB, new ItemModelComparer()).ToList();//差集
ListResult = ListA.Union(ListB, new ItemModelComparer()).ToList(); //并集
ListResult = ListA.Intersect(ListB, new ItemModelComparer()).ToList();//交集
2、 join的用法
若出现两个List，想用Linq进行查询。则可以使用如下的方式
int tempSatisfiedConditionCount = (from r1 in whiteList
join r2 in args.DiagList on r1.DiagCode equals r2.DiagCode
select r1).Count();
多条件的join
//多条件的联合查核
List<DiagControlModel> whiteDiagList = (from r1 in diagControlList
where r1.ControlRelation == 1
join r2 in args.DiagList on new { code = r1.DiagCode, name = r1.DiagName }
equals new{code=r2.DiagCode,name=r2.DiagName}
select r1).ToList<DiagControlModel>();
另外，join还可以用于左连接/右连接
var LeftJoin = from emp in ListOfEmployees
join dept in ListOfDepartment
on emp.DeptID equals dept.ID into JoinedEmpDept
from dept in JoinedEmpDept.DefaultIfEmpty()
select new
{
EmployeeName = emp.Name,
DepartmentName = dept != null ? dept.Name : null
};
3、let用法
let是一个在linq中定义局部变量
1、可以有多个let子句
2、let后的变量无需声明类型
参见下例：
var query = from a in list
let b = a.Name.Split('-')
let c=a.Age
where b[0] =="zhangs" & c>21
select a;
4、分组
常用的分组用法：
var linqtest = from r in db.Am_recProScheme
orderby r.rpId descending
group r by r.recType into n
select new
{
n.Key, //这个Key是recType
rpId = n.Sum(r => r.rpId), //组内rpId之和
MaxRpId = n.Max(r => r.rpId),//组内最大rpId
MinRpId = n.Min(r => r.rpId), //组内最小rpId
};
略微复杂点：
var dataList= (from r in drugLabelList
group r by new
{
r.OrderNo,
r.PatientID,
r.PatientName
} into q
let drugAmount = q.Sum(t => Convert.ToInt32(t.ChargeAmount))
where drugAmount >= 0
select new PrintDrugLabelModel
{
OrderNo = q.Key.OrderNo,
PatientID = q.Key.PatientID,
PatientName = q.Key.PatientName,
ChargeAmount = ""+drugAmount
}).ToList<PrintDrugLabelModel>();```
5、简单的函数计算
```java
var linqtest = (from r in db.Am_recProScheme
select r).Sum(p => p.rpId);
6、排序order by desc/asc
var linqtest = (from r in db.Am_recProScheme
where r.rpId > 10
orderby r.rpId descending //倒序
// orderby r.rpId, r.rpname descending //多条件的倒序（与SQL语句中的相同）
// orderby r.rpId ascending //正序
select r);
7、top(1)
//如果取最后一个可以按倒叙排列再取值
var linqtest = (from r in db.Am_recProScheme select r).FirstOrDefault();
8、跳过前面多少条数据取余下的数据
var linqtest = (from r in db.Am_recProScheme
where r.rpId > 10
orderby r.rpId descending
select r).Skip(10).Take(10); //取第11条到第20条数据
9、包含
//可以使用List、Array、string的Contains()方法进行判断
var linqtest = (from r in db.Am_recProScheme
where r.SortsText.Contains("张")
select r);
10、连接查询
var linqtest = (from r in db.Am_recProScheme
join w in db.Am_Test_Result on r.rpId equals w.rsId
orderby r.rpId descending
select r);
11、使用linq查询DataTable
//遍历DataTable，将其中的数据对应到ClientStruct中：
List<ClientStruct> list = (from x in dtTable.AsEnumerable()
orderby x.Field<string>("") descending
where x.Field<string>("ErrorType") == "漏孔"
select new ClientStruct
{
ID = x.Field<string>(cs.ID),
Name = x.Field<string>(cs.Name),
Company = x.Field<string>(cs.Company),
CreatedDate = x.Field<string>(cs.CreatedDate)
}).ToList<ClientStruct>();
12、linq中列传行操作（Aggregate的使用）—2022-3-4
解决分组又聚合的问题。
后台数据存储的样式是：
希望实现：
即：实现分组，并且能将诊断进行聚合（列转行）
linq的写法如下：
var data_list = (from r in temp_result_list
group r by new
{
r.DrugCode,
r.VisitType
} into q
select q.Aggregate(temp_result_list[0],(workingSentence, next) => new DiagControlModel
{
DrugCode = q.Key.DrugCode,
VisitType = q.Key.VisitType,
DiagInfoStr = workingSentence.DiagInfoStr + ";" + next.DiagName
})).ToList<DiagControlModel>();
Aggregate的具体说明，参见官网Aggregate方法说明
