Model classes and constructors for relationships

Question

Can someone help me understand what purpose the Department constructor has in the class below, taken from https://msdn.microsoft.com/en-us/data/jj591617.aspx
public class Department
public Department()
this.Courses = new HashSet<Course>();
// Primary key
public int DepartmentID { get; set; }
public string Name { get; set; }
public decimal Budget { get; set; }
public System.DateTime StartDate { get; set; }
public int? Administrator { get; set; }
// Navigation property
public virtual ICollection<Course> Courses { get; private set; }
Versus this example where they don't use a constructor, taken from http://www.asp.net/mvc/overview/getting-started/getting-started-with-ef-using-mvc/creating-a-more-complex-data-model-for-an-asp-net-mvc-application
public class Department
public int DepartmentID { get; set; }
[StringLength(50, MinimumLength=3)]
public string Name { get; set; }
[DataType(DataType.Currency)]
[Column(TypeName = "money")]
public decimal Budget { get; set; }
[DataType(DataType.Date)]
[DisplayFormat(DataFormatString = "{0:yyyy-MM-dd}", ApplyFormatInEditMode = true)]
[Display(Name = "Start Date")]
public DateTime StartDate { get; set; }
public int? InstructorID { get; set; }
public virtual Instructor Administrator { get; set; }
public virtual ICollection<Course> Courses { get; set; }
The Course class is identical in both.
Is this just a case of the latter being EF6 and the former deprecated?
Thanks
You are so wise...like a miniature budha covered in fur. -Anchorman

Answer 1

public class
Department
    public Department()
        this.Courses
= new
HashSet<Course>();
What is happening is that when Department class is instantiated into an object,  Courses gets
instantiated into a object too within Department.
var dept = new Department();
It means that Courses can be addressed/accessed immediately within Department, because it got instantiated when Department was instantiated.
dept.Courses.Add(Course);
Without Courses object being instantiated  within Department's constructor, then you would have to instantiate the Courses object outside of the Department object before you could address/access the Courses
object within the Department object.
var dept = new Department();
dept.Courses = new HashSet<Course>();
The second class example you show assumes that Courses was instantiated at some point to be an object in the Department object in code outside of the Department class.
The first class example has Courses instantiated as an object within Department and ready to go when Department object is instantiated, and the second example is you had better at some point in code outside of the Department object instantiate Courses as
an object within the Department object, because otherwise, it's object not set to an instance of an object error when you try to address Department.Courses.