Styjun

I have the following two classes (in C#)

public class courseList 

 public string MajorName get; set; 
 public string MajorNameID get; set; 
 public string CourseID get; set; 
 public string CourseName get; set; 


public class CourceNames

 [DataMember]
 public string CourseID get; set; 
 [DataMember]
 public string CourseName get; set; 


public class Courses

 [DataMember]
 public string MajorNameID get; set; 
 [DataMember]
 public string MajorName get; set; 
 [DataMember]
 public List<CourceNames> CourseNames get; set; 

 public Courses()
 
 Course = new List<CourceNames>();
 

I am reading two tables from MYSQL database using SQLreader to

List<courseList> courseList

class.
MY result record is as follows :

MajorNameID MajorName CourseName CourseID
100000 Physics Thermodynamic PHY101
100000 Physics Quantum PHY200
100000 Physics Relativity PHY300
200000 Chemistry Gases CHM300
200000 Chemistry Oreganic CHM500
200000 Chemistry Inroganic CHM120
300000 Mathematics Pure MAT100
300000 Mathematics Applied MAT300 

As u could see, I want to populate Courses class. I am not sure how I could do this using Linq.

I recently learnt the following method but it's not working correctly.

 List<Courses> courses = courseList .GroupBy(
 d => new d.MajorNameID , d.MajorName ,
 d => d.MajorName,
 (key, g) => new courses 
 
 MajorNameID = key.MajorNameID,
 MajorName = key.MajorName,
 CourseNames = g.Distinct().ToList()
 
 ).ToList();

I get the following error :

> Severity Code Description Project File Line Suppression State
> Error CS0029 Cannot implicitly convert type
> 'System.Collections.Generic.List<string>' to
> 'System.Collections.Generic.List<CourceNames>'...........

I do not fully understand the above. Could someone help me how I could set it up correctly?
I want to load couses per each major in the list.

Try simplifying your group by to just grouping by key and using Select expression on each item in group.

This final trick is that you have to cast the course names to the correct type. The compiler should give you enough hints, but Linq GroupBy is VERY different to MySQL/TSQL GroupBy statements so they can be tricky to master at first.

List<Courses> courses = courseList.GroupBy(
 d => new d.MajorNameID, d.MajorName 
 ).Select(g => new Courses
 
 MajorNameID = g.Key.MajorNameID,
 MajorName = g.Key.MajorName,
 CourseNames = g.Distinct().Select(c => 
 new CourceNames CourseID = c.CourseID, CourseName = c.CourseName ).ToList()
 
).ToList();

[UPDATE: I don't normally use element projection syntax, so I had to compile this to check]
Using the same GroupBy overload as your question (element projection), this is the same query, note that the element selector expression now selects the elements, the original post has a key expression in there instead:

List<Courses> courses2 = courseList.GroupBy(
 d => new d.MajorNameID, d.MajorName , // key selector
 c => new CourceNames CourseID = c.CourseID, CourseName = c.CourseName , // element selector
 (key, g) => new Courses
 
 MajorNameID = key.MajorNameID,
 MajorName = key.MajorName,
 CourseNames = g.Distinct().ToList()
 
).ToList();

The lambda variable g is a group, you cannot directly do a .Distinct on it

List<Courses> courses = courseList .GroupBy(
 d => new d.MajorNameID , d.MajorName ,
 d => d.MajorName,
 (key, g) => new Courses 
 
 MajorNameID = key.MajorNameID,
 MajorName = key.MajorName,
 CourseNames = g.Distinct().ToList()
 ).ToList();

You need to project the course name from the group g and then do a .Distinct on it. Which will be like:

List<Courses> courses = courseList .GroupBy(
 d => new d.MajorNameID , d.MajorName ,
 d => d.MajorName,
 (key, g) => new Courses 
 
 MajorNameID = key.MajorNameID,
 MajorName = key.MajorName,
 CourseNames = g.Select(x => x.CourseName).Distinct().ToList()
 ).ToList();