HashSet in EF many to many

Question

I'm trying to implement many to many in EF Code-first. I have found this code :

 public class Student
    {
        public Student() { }

        public int StudentId { get; set; }
        [Required]
        public string StudentName { get; set; }

        public virtual ICollection<Course> Courses { get; set; }
    }

    public class Course
    {
        public Course()
        {
            this.Students = new HashSet<Student>();
        }

        public int CourseId { get; set; }
        public string CourseName { get; set; }

        public virtual ICollection<Student> Students { get; set; }
    }

I understand everything except :

    public Course()
    {
        this.Students = new HashSet<Student>();
    }

Can you tell me why this part is necessary? Thanks.

Answer 1

It's necessary because you have to instantiate the particular implementation of ICollection that you would like to use. HashSet implements a hash table that is very efficient for a lot of operations, for instance searching a large set for a single item. But you might have reasons for choosing some other implementation, like List. It is equally fine to instantiate the collection as this.Students = new List<Student>(); - Entity Framework doesn't care, but the default is HashSet for efficiency reasons.