Save a List of objects in another object.

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.

"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!

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.

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.

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.
            }  

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?

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!