LINQ query with Distinct and WHERE clause

Question

My Distinct query is working fine, but I need to put a WHERE clause in another method.

This is my working query for retrieving distinct managers

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. But with WHERE clause I get "Enumeration yielded no results"

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"
}

Answer 1

What you are doing is sending a reqest to the db and when the request is finished you do your groupby etc.

so when the db return nothing you get the exception

here is how i would do it

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;
    }

Answer 2

I would do it

 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

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

 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"
    }