Creating objects from stored procedure result

Question

We have an existing SQL Server stored procedure with following query. We need to create a collection of Student objects in the following class design, from the result of the query.

What is the best way to create the objects from the SqlDataReader using LINQ?

Note: I am using SqlDataReader only; no ORM

Query

SELECT 
    S.StudentID, S.StudentName, E.ExamID, E.ExamName, SE.Mark 
FROM 
    StudentExam SE
INNER JOIN 
    Student S ON S.StudentID = SE.StudentID
INNER JOIN 
    Exam E ON E.ExamID = SE.ExamID 

Class

public class ExamMark
{
    public int ExamID { get; set; }
    public string ExamName { get; set; }
    public int Mark { get; set; }
}

public class Student
{
    public int StudentID { get; set; }
    public string StudentName { get; set; }
    public List<ExamMark> examResults { get; set; }
}

SqlDataReader

   SqlDataReader reader = command.ExecuteReader();
   if (reader.HasRows)
   {
      while (reader.Read())
      {

      }
   } 

REFERENCES

1. LINQ: Fill objects from left join
2. Complex GROUP BY on DataTable

Answer 1

Well, I wouldn't do it like that,

I'd have two statements

-- Student Statement
SELECT
             S.StudentID,
             S.StudentName
    FROM
             Student S
    WHERE
             EXISTS (
              SELECT * FROM StudentExam SE WHERE SE.StudentID = S.Student.ID);

-- Exam Statement
SELECT
            SE.StudentID,
            E.ExamID,
            E.ExamName,
            SE.Mark 
    FROM
            StudentExam SE
        JOIN
            Exam E
                ON E.ExamID = SE.ExamID;

Then, I'd have a function that does this,

private IEnumerable<Tuple<int, ExamMark>> GetMarks()
{
    ... setup the exam command here
    var reader = examCommand.ExecuteReader();
    while (reader.Read())
    {
        yield return Tuple.Create(
            reader.GetInt32(0),
            new ExamMark
                {
                    reader.GetInt32(1),
                    reader.GetString(2),
                    reader.GetInt32(3)
                });
    }
}

Then I'd have this function to call,

private IEnumerable<Student> GetStudents()
{
    var resultLookup = GetMarks().ToLookup(t => t.Item1, t => t.Item2);

    ... setup the student command here
    var reader = studentCommand.ExecuteReader();
    while (reader.Read())
    {
        var studentId = reader.GetInt32(0);
        yield return new Student
                {
                    studentId,
                    reader.GetString(1),
                    resultLookup[studentId].ToList()
                });
    }
}

---

If you wanted, you could do it all in one stored procedure and return multiple result sets.

Answer 2

This should do the work:

using (SqlDataReader reader = command.ExecuteReader())
        {
            var records = (from record in reader.Cast<DbDataRecord>()
                           select new
                           {
                               StudentID = record.GetInt32(0),
                               StudentName = record.GetString(1),
                               ExamID = record.GetInt32(2),
                               ExamName = record.GetString(3),
                               Mark = record.GetInt32(4)
                           })
                           .GroupBy(r => new { StudentID = r.StudentID, StudentName = r.StudentName })
                           .Select(
                                     r => new Student
                                     {
                                         StudentID = r.Key.StudentID,
                                         StudentName = r.Key.StudentName,
                                         examResults = r.Select(e => new ExamMark
                                         {
                                             ExamID = e.ExamID,
                                             ExamName = e.ExamName,
                                             Mark = e.Mark
                                         }).ToList()
                                     });
        }