return type of linq group by multiple column

Question

I have the following code:

 var studentClassAttendancesGrouped =
                            classAttendanceByCriteria.OrderBy(x => x.Lecture.Id).GroupBy(x => new { x.Lecture, x.Teacher });

I am trying to return studentClassAttendancesGroupedfrom a method.But I can not determine the return type of the method.

Answer 1

This overload of the GroupBy will return IEnumerable<IGrouping<TKey, TSource>> as seen from the source code.

You just need to project your enumerable with the Select() and of course you must call ToList() at the end to execute the query and create a list:

var res = classAttendanceByCriteria.OrderBy(x => x.Lecture.Id)
                                   .GroupBy(x => new { x.Lecture, x.Teacher })
                                   .Select(x => new {x.Key.Lecture, x.Key.Teacher})
                                   .ToList();

But, the above code creates a list of the anonymous types. If you are going to return the above statement from the method, then you need to use or to create a custom object and return the list of this:

public class LectureTeacher
{
    public string Lecture { get; set; }
    public string Teacher { get; set; }
}

public List<LectureTeacher> GetDogsWithBreedNames()
{
    var res = classAttendanceByCriteria.OrderBy(x => x.Lecture.Id)
                                       .GroupBy(x => new { x.Lecture, x.Teacher })
                                       .Select(x => new LectureTeacher {Lecture  = x.Key.Lecture, Teacher  = x.Key.Teacher})
                                       .ToList();

    return res;
}

By the way, GroupBy has several other overloads which you can specify result selector and it will return IEnumerable to you:

myList.OrderBy(x => x.Lecture.Id) 
      .GroupBy(x => new { x.Lecture, x.Teacher } //key selector
               key => new LectureTeacher // result selector
               {
                    Lecture  = key.Lecture, 
                    Teacher  = key.Teacher
               }) 
      .ToList();

Answer 2

I would suggest you to create a custom class and return it from your method:-

IEnumerable<CustomType> studentClassAttendancesGrouped =
       classAttendanceByCriteria.OrderBy(x => x.Lecture.Id)
       .GroupBy(x => new { x.Lecture, x.Teacher })
      .Select(x => new CustomType { Lecture = x.Key.Lecture, Teacher = x.Key.Teacher });

Where, CustomType will look something like this:-

public class CustomType 
{
   public string Lecture {get; set; }
   public string Teacher {get; set; }
}

You can add more columns as per your need and finally return IEnumerable<CustomType> from your method.

Thus, your method will look like:-

public IEnumerable<CustomType> MyMethod()
{
    return classAttendanceByCriteria.OrderBy(x => x.Lecture.Id)
       .GroupBy(x => new { x.Lecture, x.Teacher })
       .Select(x => new CustomType { Lecture = x.Key.Lecture, Teacher = x.Key.Teacher });
}