Common usage of LINQ in C ＃

1、linq Middle intersection 、 Combine 、 The use of difference sets

Simple intersection 、 Combine 、 The difference set is used as follows ：

List<string> ListA = new List<string>();
List<string> ListB = new List<string>();
List<string> ListResult = new List<string>();

ListResult = ListA.Distinct().ToList();// duplicate removal 
ListResult = ListA.Except(ListB).ToList();// Difference set 
ListResult = ListA.Union(ListB).ToList();  // Combine 
ListResult = ListA.Intersect(ListB).ToList();// intersection 

If the above example is not List<string> type , It is List<XXXModel>, You need to XXXModel To deal with .
Steps are as follows ：
(1) First define Model.

    public class ItemModel
    {
    
        public string ItemCode {
     get; set; }
        public string ItemName {
     get; set; }
    }

(2) How to define Model How to compare . If not defined , Compare two references .

public class ItemModelComparer : IEqualityComparer<ItemModel>
    {
    
        // Compare 
        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;

        }
    
        // Get the hash value 
        public int GetHashCode(ItemModel model)
        {
    
            if (Object.ReferenceEquals(model, null)) return 0;

            int hashNurse = model.ItemCode.GetHashCode();

            return hashNurse;
        }
    
    }

(3) The specific use is as follows

List<ItemModel> ListA = new List<ItemModel>();
List<ItemModel> ListB = new List<ItemModel>();
List<ItemModel> ListResult = new List<ItemModel>();

ListResult = ListA.Distinct(new ItemModelComparer()).ToList();// duplicate removal 
ListResult = ListA.Except(ListB, new ItemModelComparer()).ToList();// Difference set 
ListResult = ListA.Union(ListB, new ItemModelComparer()).ToList();  // Combine 
ListResult = ListA.Intersect(ListB, new ItemModelComparer()).ToList();// intersection 

2、 join Usage of

If there are two List, Want to use Linq The query . You can use the following method

int tempSatisfiedConditionCount = (from r1 in whiteList
                                   join r2 in args.DiagList on r1.DiagCode equals r2.DiagCode
                                   select r1).Count();

Multi conditional join

// Multi conditional joint audit 
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>();

in addition ,join It can also be used for left connection / The right connection

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 usage

let It's a linq Define local variables in
1、 There can be multiple let Clause
2、let The variable after does not need to declare a type
See the following example ：

var query = from a in list  
                        let b = a.Name.Split('-')  
                        let c=a.Age  
                        where b[0] =="zhangs" & c>21  
                        select a; 

4、 grouping

Common grouping usage ：

      var linqtest = from r in db.Am_recProScheme
                     orderby r.rpId descending
                     group r by r.recType into n
                     select new
                     {
                         n.Key,  // This Key yes recType
                         rpId = n.Sum(r => r.rpId), // Within the group rpId The sum of the 
                         MaxRpId = n.Max(r => r.rpId),// The largest in the group rpId
                         MinRpId = n.Min(r => r.rpId), // The smallest in the group rpId
                     };

A little more complicated ：

	             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、 Simple function calculation

      var linqtest = (from r in db.Am_recProScheme
                      select r).Sum(p => p.rpId);

6、 Sort order by desc/asc

     var linqtest = (from r in db.Am_recProScheme
                     where r.rpId > 10
                     orderby r.rpId descending  // In reverse order 
                     // orderby r.rpId, r.rpname descending   // The reverse order of multiple conditions （ And SQL Statement ）
                     //  orderby r.rpId ascending   // positive sequence 
                     select r);

7、top(1)

            // If you take the last one, you can arrange the values according to the flashback 
      var linqtest = (from r in db.Am_recProScheme                     
                      select r).FirstOrDefault();

8、 Skip the previous data and get the remaining data

      var linqtest = (from r in db.Am_recProScheme
                      where r.rpId > 10
                      orderby r.rpId descending
                      select r).Skip(10).Take(10); // Take the first place 11 To the first article 20 Data   

9、 contain

	   // have access to List、Array、string Of Contains() Method to judge 
     var linqtest = (from r in db.Am_recProScheme
                     where r.SortsText.Contains(" Zhang ")
                     select r);

10、 Link query

      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、 Use linq Inquire about DataTable

            // Traverse DataTable, Map the data to ClientStruct in ：  
            List<ClientStruct> list = (from x in dtTable.AsEnumerable()  
                                       orderby x.Field<string>("")  descending  
                                       where x.Field<string>("ErrorType") == " Leak " 
                                       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 Middle column row transfer operation （Aggregate Use ）—2022-3-4

Solve the problem of grouping and aggregation .
The style of background data storage is ：

Hope to achieve ：

namely ： Implement grouping , And can aggregate diagnostics （ Column turned ）
linq Let's write it as follows ：

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 Specific description of , See Official website Aggregate Method statement

Reference resources ： Zhanglonghao's blog in the blog park