Save a List of objects in another object.

Question

I have a Customer-object and a Book-object in a library-system.

When I register a loan I want the chosen Book-object to be saved to a List-control in the Customer-object.

The idea is to show all the books a customer have lended in a richTextBox.

I did this in an old Java-course and I have been trying to translate it to C#.

In the Customer-Class I have declared:
private List<Bok> loan;
Have a property that returns a booklist:
public list<Book> loan
{
get
{
return loan = new List<Book>();
}
} I also instanciates the List in a constructor.
Then I do this when try to register the loan:
bookToLend.BiblioteksKund = lender; //bookToLend is a Book-object.
lender.Loan.Add(bookToLend); //lender is a Customer-object, loan is a List of Books.
I get no error but the selected book doesn't seem to be added to the list in the customer-object. Why?

Vulpes · Answer

The problem is that you're returning a new List object every time the Loan property is accessed. You just need to return the existing object: public class Customer { private List loan = new List (); public List Loan { get { return loan; } } }

Sam Hobbs · Answer

Yes a control is usually a UI thing only. You could easily confuse people by saying control if it is not something for a window (form or whatever). It is not easy to decide when to say borrow or borrowed instead of lend or lent. The important thing is to know about the word borrow and to understanding its relationship with lending. There is a member in this community that I will not name but I know he is definitely a native to the USA and English is his primary and probably only language but his grammer is quite different from most people whose native language is English. So yes grammer is difficult to get right. I hope it helps to have help with some of the confusion.

Lars Persson · Answer

You said in the original question Book-object to be saved to a List-control in the Customer-object. I hope that what you are actually doing is saving the book in a Customer object and showing in a control.  Hm not really sure what you mean here. I save a book-object in a List (perhaps control isn't what you call it ) that I have declared in my customer-class. This way I know what books the customer have borrowed . When I look up a customer I show the borrowed books in a ritchTextBox. Is there a better way? Finaly note that the question about can´t lend a book that is already been lent seems to be a new question; I think it is independent of the original question. Yes but it was about the same issue saving books in a customer-object so I decided to keep it in the same thread. This is about judgment I suppose and maybe I made a wrong one. About lending and borrowing it is confusing. Didn't think of the the borrow actually but in my case I don't think it matters. Thanks for clearing that up though!

Vulpes · Answer

Well lending (or loaning) and borrowing are of course two sides of the same transaction so I don't think it matters all that much what names Lars uses for his variables. However I'm not surprised that he's confused by these words. There are plenty of native English speakers in the UK who would say they're going to 'lend a book from the library' when they mean they're going to borrow it.

Sam Hobbs · Answer

You said in the original question Book-object to be saved to a List-control in the Customer-object. I hope that what you are actually doing is saving the book in a Customer object and showing in a control. One of the purposes of the code I posted for you in the other thread is so you can see the concept of keeping the data separate from the UI and then using the UI just as a UI. Microsoft is teaching developers bad programming by putting everything in a Form class. Developers tend to also do that by using the form as a container of the data then they must figure out how to share the data across forms. When more than one form is needed it helps to not use a form as the palce to keep the data. You say yo are nto sure of the proper grammer. It is interesting that Vulpes is using grammer consistent with your use. Vulpes of course is familiar with the term borrow. A library lends books that are borrowed by people. So a lender is a library. When a book is lent by a libray it is borrowed by the customer or member. So this is one area where English can be confusing. Finaly note that the question about can´t lend a book that is already been lent seems to be a new question; I think it is independent of the original question.

Lars Persson · Answer

Well you are right that if I don´t have a customer-object I of course can´t send it to the book-object. I would get an exception. That however wasn´t my problem. When I tested I always had a customer-object and if I didn´t the code would break. The problem was that i didn´t clear the richTextBox where I present the books belonging to the customer. So when I updated the customer the same book would be added in the richTextBox. As it turned out the code worked fine. Thanks though for the error you pointed out.

Vulpes · Answer

If the customer is not found (i.e. lender == null) then I think you need to avoid executing the rest of the code either by returning from whatever method you're in (I imagine an eventhandler) or making the execution of the rest of the code conditional on lender not being null. If you don't do that then if the book exists and is not lent the libraryCustomer property would be reset to null and you'd then get an exception when you try to add the book to the customer's Loan collection. Similarly if the book is not found you also don't want to execute the rest of the code to avoid an exception when bookToLend is null. //Search for chosen customer foreach (Kund K in CustomerList) //Kund is customer in swedish... { //if customer found if (txtPersonNr.Text.Equals(K.PersonNr.ToString())) lender = K; //Kunden som hittats läggs i ny variabel. } //Om customer is not found if (lender == null) { textBox7.Text = Bok eller Kund saknas!; return; // return from method here } //Search for chosen book foreach (Bok B in BookList) { //Om chosen book is found if (textBox6.Text.Equals(B.ISBN.ToString())) bookToLend = B; } //Om chosen book is not found if (bookToLend == null) { textBox7.Text = Bok eller Kund saknas!; return; // return from method here } if (bookToLend.libraryCustomer == null) //Om bok inte redan är utlånad { bookToLend.libraryCustomer = lender; //tilldelar vald kund till bok. lender.Loan.Add(bookToLend); //Lägger lånad bok i kundens bokLista. }

Lars Persson · Answer

Now that I can save several books that the Customer lends I want to make sure that you can´t lend a book that is already been lent. (not sure if that is correct english...). When a Customer lends a book an object of type Customer-class is set in the book-class. Now we know that the book has been lent and to lent it (the Customer object). I set the variable pointing at null so if it is still null no one has lended the book. So I check to see if the variable in the book-object is null or not. Kund lender = null; Bok bookToLend = null; //Search for chosen customer foreach (Kund K in CustomerList) //Kund is customer in swedish... { //if customer found if (txtPersonNr.Text.Equals(K.PersonNr.ToString())) lender = K; //Kunden som hittats läggs i ny variabel. } //Om customer is not found if (lender == null) textBox7.Text = Bok eller Kund saknas!; //Search for chosen book foreach (Bok B in BookList) { //Om chosen book is found if (textBox6.Text.Equals(B.ISBN.ToString())) bookToLend = B; } //Om chosen book is not found if (bookToLend == null) textBox7.Text = Bok eller Kund saknas!; if (bookToLend.libraryCustomer == null) //Om bok inte redan är utlånad { bookToLend.libraryCustomer = lender; //tilldelar vald kund till bok. lender.Loan.Add(bookToLend); //Lägger lånad bok i kundens bokLista. } The idea is this: If the variable of type customer-class is still null (libraryCustomer) no one has lended the book. If so the chosen customer-object is set in the book object. Now no one else can lend the book and you can't loan it twice. The problem is that you can loan it twice. What am I missing here?

Lars Persson · Answer

Thanks! I think did this at one point but probably made another error. When you think about it it is quite logic. Now it works!