public class Department
{
    public int DepartmentId { get; set; }

    public string DepartmentName { get; set; }

    public ICollection<Employee> Employees { get; set; }
}

public class Employee
{
    public Guid EmployeeId { get; set; }

    public string Name { get; set; }
}

public class App
{
    static int main()
    {
        HashSet<Department> sut = new HashSet<Department>()
        {
            new Department()
            {
                DepartmentId = 1,
                DepartmentName = "左A",
                Employees = new []
                {
                    new Employee
                    {
                        EmployeeId = Guid.Parse("becc484a-d8f7-4569-ad35-720fe7a454fb"),
                        Name = "Jason",
                    }
                }
            }
        };

        Console.WriteLine(sut.DumpToString());
    }
}

"new HashSet<Department>()\r\n{\r\n\tnew Department()\r\n\t{\r\n\t\tDepartmentId = 1,\r\n\t\tDepartmentName = \"左A\",\r\n\t\tEmployees = new[]\r\n\t\t{\r\n\t\t\tnew Employee()\r\n\t\t\t{\r\n\t\t\t\tEmployeeId = Guid.Parse(\"becc484a-d8f7-4569-ad35-720fe7a454fb\"),\r\n\t\t\t\tName = \"Jason\",\r\n\t\t\t},\r\n\t\t},\r\n\t},\r\n}"