[英]LINQ query with Distinct and WHERE clause
My Distinct query is working fine, but I need to put a WHERE clause in another method. 我的Distinct查询工作正常,但是我需要在另一个方法中放入WHERE子句。
This is my working query for retrieving distinct managers 这是我检索不同经理的工作查询
public static IEnumerable<DTO.Employee> GetDistinctMgrs()
{
IEnumerable<DTO.Employee> managers = EmpCache.EmployeeList.GroupBy(x => x.MgrID).Select(x => x.FirstOrDefault());
return managers;
}
Here is where I am trying to squeeze WHERE to get employee's manager from distinct list of managers. 这是我试图从何处从不同的经理列表中选拔员工经理的地方。 But with WHERE clause I get "Enumeration yielded no results"
但是有了WHERE子句,我得到“枚举没有结果”
public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
.Where(s => s.EmpID == EMPID)
.GroupBy(x => x.MgrID)
.Select(x => x.FirstOrDefault());
return manager; //<- "Enumeration yielded no results"
}
What you are doing is sending a reqest to the db and when the request is finished you do your groupby etc. 您正在做的是向数据库发送请求,当请求完成时,您进行groupby等。
so when the db return nothing you get the exception 因此,当数据库不返回任何内容时,您将获得异常
here is how i would do it 这是我会怎么做
public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
.Where(s => s.EmpID == EMPID)
.GroupBy(x => x.MgrID);
if(manager?.Count()>0)
return manager.Select(x => x.First());
return null;
}
I would do it 我会做
public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
IEnumerable<DTO.Employee> manager =
from mng in EmpCache.EmployeeList
where mng.EmpID.ToLower() == EMPID.ToLower()
orderby mng.MgrID
select mng;
return manager;
}
or 要么
public static IEnumerable<DTO.Employee> GetDefaultMgr(string EMPID)
{
IEnumerable<DTO.Employee> manager = EmpCache.EmployeeList
.Where(s => s.EmpID.ToLower() == EMPID.ToLower())
.GroupBy(x => x.MgrID)
.Select(x => x.FirstOrDefault()).ToList();
return manager; //<- "Enumeration yielded no results"
}
Based in your new question 根据您的新问题
public static string GetDefaultMgr(string EMPID)
{
string managerID = EmpCache.EmployeeList
.Where(s => s.EmpID.ToLower() == EMPID.ToLower())
.GroupBy(x => x.MgrID)
.Select(x => x.MgrID).FirstOrDefault().ToString();
return managerID; //<- "Enumeration yielded no results"
}
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