/// <summary>
        /// Reads all departments not assigned to an Employee
        /// </summary>
        /// <returns>The returned collection</returns>
        public IEnumerable DepartmentsNotAssigned()
        {
            var allEmployeeIds = EmployeeData.Select(emp => emp.DepartmentId).ToList();

            return(DepartmentData.Where(dept => allEmployeeIds.Contains(dept.DepartmentId) == false));
        }