/// <summary>
/// verificate if the employee has actived contracts, if it is true, it we can not delete him
/// </summary>
/// <param name="oldEmployee"></param>
public void DelEmployee(Employee oldEmployee)
{
IEnumerable <Contract> verification = from contract in dal.ReturnContracts()
where contract.employeeId == oldEmployee.id && contract.hasBeenSigned
select contract;
if (verification.Any(contract => contract.endWork < DateTime.Now))
{
throw new Exception("We can not delete employee because he has actived contracts");
}
dal.DelEmployee(oldEmployee);
}