public class Employee
{
    public int ID { get; set; }

    public string fname { get; set; }

    public string lastname { get; set; }

    public decimal? salary { get; set; }

    public virtual Department department { get; set; }

    public string address { get; set; }

    [EmailAddress]
    [DefaultValue("ahmet@yahoo.com")]
    public string? email { get; set; }

    [Range(18, 60)]
    public int age { get; set; }
}
public class Department
{
    public int DepartmentID { get; set; }

    public string name { get; set; }

    public int yearofcreation { get; set; }

    public ICollection<Employee> employees { get; set; } = new HashSet<Employee>();

}
[Table("courses")]
public class TrainingCourse
{
    [Key]
    public int coursenumber { get; set; }

    [Required]
    [MaxLength(50)]
    public string coursetitle { get; set; }

    [Column("duration", TypeName = "int")]
    public short courseduration { get; set; }

    [Required]
    public string courseurl { get; set; }

    public virtual ICollection<Employee> Employees { get; set; } = new HashSet<Employee>();


    public virtual ICollection<CourseEmployee> courseEmployees { get; set; }
}
using (mydbcontext context=new mydbcontext())//using()block for disbosing(removing unmanged resource)
        {   
            Department dp1=new Department(){name="engineering"};
            Department dp2=new Department(){name="Hr"};
           
            Employee emp=new Employee(){fname="ahmed",lastname="eid",address="alex"};
            
            context.Departments.Add(dp1);//or
            context.Add(dp2);
            context.Add(course);
            context.Employees.Add(emp);
            context.SaveChanges();
            Console.WriteLine(dp1.name);
            
            var results=context.Departments.Where(d=>d.name=="Hr");
            Department d=results.FirstOrDefault();
            

        }