Basic OOPS..invariant and object creation

kaspyanand · May 23rd, 2006, 04:13 AM

The requirement in my project is creation of itinearay ..which will have a name,category(like adventure,classisc),noofdays(eg 6 day itinerary)..so..to design this situation when user creates an itinerary,i need to create an itinerary object with name,category and noofdays(my understnding is that these are invariant of class)..my question is where to check if data supplied by user is incorrect(eg 0 nofodays is invalid)..in constructor..if so,if data is invalid..what nees to be done raise exception and destroy object? or use a factory class to create object..which will check data provided and if everything is ok,then create itinerary object..How i have implememted..

Code:

public class Itinerary
    {
        private string _ItineraryName;
        private byte _ItineraryDays;
        private string _ItineraryCategory;

     public  Itinerary(string itineraryName,byte itineraryDays,string itineraryCategory)
        {
            if (itineraryName == "")
            {
                throw new ArgumentException() ; 

            }
            else
            {
                _ItineraryName = itineraryName;
                _ItineraryDays = itineraryDays;
                _ItineraryCategory = itineraryCategory;
            }
        }

.

bijgupt · May 24th, 2006, 05:58 AM

within constructor's else part

Bijgupt

braindead · May 24th, 2006, 06:04 PM

The answer to this question depends on the structure of your code.

I would suggest checking that the data is valid before creating the new Instance (and thus calling the constructor).

Where is the data that is sent to the constructor coming from? If it is coming from form controls, then
I would suggest something like this:

// assuming all controls are textboxes:
Itinerary itinerary = null;
string IName = ItineraryNameControl.Text;
int IDays;
if (!TryParse(ItineraryDaysControl.Text, IDays))
   IDays = 0;
string ICategory = ItineraryCategory.Text;

if (IName.Equals(String.Empty))
   MessageBox.Show ("Need name of itinerary.");
else if (IDays == 0)
   MessageBox.Show ("Please provide the number of days.");
else if (ICategory.Equals(String.Empty))
   MessageBox.Show ("Please categorize this itinerary.");
else
{
   itinerary = new Itinerary (IName, IDays, ICategory);
   // process away.
}

In this way, you know that the instance will be constructed with valid data.

Brandon