C＃中Linq常用用法

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

若上面的例子不是List<string>类型，而是List<XXXModel>，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)
                {
    
                    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();

            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

      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

参考：博客园的张龙豪博客