/// <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));
}