Saving objects with Cross-reference
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Hi! I made a library-project in C#. I save the customers in one list and the books in one. When I register a that a Customer borrows a book this is what happends: (Sorry for my english, I am Swedish) First the customer (lender) is assigned to the book, so the book "knows" who has borrowed it. bookToLend.BiblioteksKund = lender; Then the book is added to a List-control owned by the customer-object so the customer-object remembers what books the customer has borrowed. This works fine when the program is running. The problem occurs when I try to save the objects to disc and then retrieve them again when the program starts. I have asked for help and googled it but I can´t make it work. The problem seems to be the cross-reference of objects. I´ll be happy to send the project if someone wants to look at it. I have tried to use Serialization but have not succeeded. Perhaps Linq would be a better choise.
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When you are saving objects with cross reference, then always be sure to choose the right format.
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List BookList = new List(); List CustomerList = new List(); static string dir = @"C:\\Bibliotek\\"; string path = dir + "Kunder.xml"; string path2 = dir + "Books.xml"; XmlSerializer xs = new XmlSerializer(typeof(List)); XmlSerializer xs2 = new XmlSerializer(typeof(List));public void sparaKund() //Save customerlist
{using (Stream s = File.Create(path)) { xs.Serialize(s,CustomerList); } using (Stream s2 = File.Create(path2)) { xs2.Serialize(s2, BookList); } }The file is created but is empty.
private void Form1_Load(object sender, EventArgs e) //Load customerlist
{// deserialize to a new list if (File.Exists(path)) { using (Stream s = File.OpenRead(path)) { CustomerList = (List)xs.Deserialize(s); } } if (File.Exists(path2)) { using (Stream s2 = File.OpenRead(path2)) { BookList = (List)xs2.Deserialize(s2); } } }Looks good enough for me. Can you now post the definition of the Kund and Book-classes? I hope that each book has a unique number, or GUID, and I hope to see a list of those numbers in the Kund-object that borrowed them. My guess is that you're saving the lists without the references.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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Looks good enough for me. Can you now post the definition of the Kund and Book-classes? I hope that each book has a unique number, or GUID, and I hope to see a list of those numbers in the Kund-object that borrowed them. My guess is that you're saving the lists without the references.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Well, you set the unique number when you register a book (Bok) or a customer (kund). It's not a Commercial application.
Book:
[Serializable()]
public class Bok : ISerializable{
protected int isbn = 0;
protected string titel = null;
protected string author;
protected int price;
protected String isType = null;
protected Kund biblioteksKund = null;//egenskaper (properties). Används för att förhindra åtkomst till privata variabler. public int ISBN { get { return isbn; } set { isbn = value; } } public string Titel { get { return titel; } set { titel = value; } } public string Author { get { return author; } set { author = value; } } public int Price { get { return price; } set { price = value; } } public String IsType { get { return isType; } set { isType = value; } } public Kund BiblioteksKund { get { return biblioteksKund; } set { biblioteksKund = value; } } //Konstruktor (constructor) Metod som körs när objektet skapas. Denna körs om det inte finns parametrar. public Bok() { } //Konstruktor med parametrar public Bok(int isbn, string titel, string author, int price, SerializationInfo info, StreamingContext ctxt) { ISBN = isbn; Author = author; Titel = titel; Price = price; biblioteksKund = new Kund(); this.isbn = (int)info.GetValue("ISBN", typeof(int)); this.author = (string)info.GetValue("Author", typeof(string)); this.titel = (string)info.GetValue("Titel", typeof(string)); this.price = (int)inf -
Well, you set the unique number when you register a book (Bok) or a customer (kund). It's not a Commercial application.
Book:
[Serializable()]
public class Bok : ISerializable{
protected int isbn = 0;
protected string titel = null;
protected string author;
protected int price;
protected String isType = null;
protected Kund biblioteksKund = null;//egenskaper (properties). Används för att förhindra åtkomst till privata variabler. public int ISBN { get { return isbn; } set { isbn = value; } } public string Titel { get { return titel; } set { titel = value; } } public string Author { get { return author; } set { author = value; } } public int Price { get { return price; } set { price = value; } } public String IsType { get { return isType; } set { isType = value; } } public Kund BiblioteksKund { get { return biblioteksKund; } set { biblioteksKund = value; } } //Konstruktor (constructor) Metod som körs när objektet skapas. Denna körs om det inte finns parametrar. public Bok() { } //Konstruktor med parametrar public Bok(int isbn, string titel, string author, int price, SerializationInfo info, StreamingContext ctxt) { ISBN = isbn; Author = author; Titel = titel; Price = price; biblioteksKund = new Kund(); this.isbn = (int)info.GetValue("ISBN", typeof(int)); this.author = (string)info.GetValue("Author", typeof(string)); this.titel = (string)info.GetValue("Titel", typeof(string)); this.price = (int)inflarsp777 wrote:
It's not a Commercial application.
That makes no difference.
larsp777 wrote:
Well, you set the unique number when you register a book (Bok) or a customer (kund).
What does 'skrivUt' mean? There's no list on "who" borrowed "wich" book. Remember that a ISBN-number doesn't idenitfy a specific book, but a "title". One could have multiple copies of the same title :)
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
It's not a Commercial application.
That makes no difference.
larsp777 wrote:
Well, you set the unique number when you register a book (Bok) or a customer (kund).
What does 'skrivUt' mean? There's no list on "who" borrowed "wich" book. Remember that a ISBN-number doesn't idenitfy a specific book, but a "title". One could have multiple copies of the same title :)
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Well, since it is run in a controlled environment I can make sure that there is only one copy with a specific ISBN. Otherwise you are right of course, ISBN identyfies a title, not a specific book. No, there is no list of who borrowed which book but each book-object "knows" who borrowed it with a reference to a customer object. protected Kund biblioteksKund = null; SkrivUt means "print" and is simply printing data about a customer but I don't think is is ever used here. (It was initially a assignment from my University made in Java.) The reason I mentioned that it is not a Commercial application was that I could made sure that no two objects are the same. But maybe that doesn't matter. Shold I use something like a GUID?
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Well, since it is run in a controlled environment I can make sure that there is only one copy with a specific ISBN. Otherwise you are right of course, ISBN identyfies a title, not a specific book. No, there is no list of who borrowed which book but each book-object "knows" who borrowed it with a reference to a customer object. protected Kund biblioteksKund = null; SkrivUt means "print" and is simply printing data about a customer but I don't think is is ever used here. (It was initially a assignment from my University made in Java.) The reason I mentioned that it is not a Commercial application was that I could made sure that no two objects are the same. But maybe that doesn't matter. Shold I use something like a GUID?
larsp777 wrote:
The reason I mentioned that it is not a Commercial application was that I could made sure that no two objects are the same. But maybe that doesn't matter. Shold I use something like a GUID?
I dunno, and this is the place where things get complicated. Let's say I borrowed Pratchetts' book "Small Gods". You have three books of this story - how are you gonna track each book you lent? Answer; give every book a unique number (yeah, like a
GUID) :)Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
The reason I mentioned that it is not a Commercial application was that I could made sure that no two objects are the same. But maybe that doesn't matter. Shold I use something like a GUID?
I dunno, and this is the place where things get complicated. Let's say I borrowed Pratchetts' book "Small Gods". You have three books of this story - how are you gonna track each book you lent? Answer; give every book a unique number (yeah, like a
GUID) :)Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Eddy Vluggen wrote:
dunno, and this is the place where things get complicated.
Let's say I borrowed Pratchetts' book "Small Gods". You have three books of this story - how are you gonna track each book you lent?
Answer; give every book a unique number (yeah, like aGUID) :)Yes, but as I said it's a controlled environment where I make sure that there is only one copy of each book. The question is if this actually is the reason to why it doesn´t work? Or could it be that the list I am trying to save is a empty list even if I use the same name? This is the event for the button where I register the loan.
private void btnRegister_Click(object sender, EventArgs e)
{
Kund lender = null;
Bok bookToLend = null;//Search for chosen customer foreach (Kund K in CustomerList) { //Om rätt kund hittas if (txtPersonNr.Text.Equals(K.PersonNr.ToString())) lender = K; //Kunden som hittats läggs i ny variabel. } //If customer wasn't found. if (lender == null) { textBox7.Text = "Kund saknas!"; return; } //Search for chosen book. foreach (Bok B in BookList) { //Om rätt bok hittas if (textBox6.Text.Equals(B.ISBN.ToString())) bookToLend = B; } //if book wasn´t found. if (bookToLend == null) { textBox7.Text = "Bok saknas!"; return; } if (bookToLend.BiblioteksKund == null) //If book doesn´t have its customer object set. { bookToLend.BiblioteksKund = lender; //Sets the customerobject of the book. lender.Loan.Add(bookToLend); //Places the book to lend in the customers booklist. textBox7.Text = "Lån registrerat!"; } } -
Eddy Vluggen wrote:
dunno, and this is the place where things get complicated.
Let's say I borrowed Pratchetts' book "Small Gods". You have three books of this story - how are you gonna track each book you lent?
Answer; give every book a unique number (yeah, like aGUID) :)Yes, but as I said it's a controlled environment where I make sure that there is only one copy of each book. The question is if this actually is the reason to why it doesn´t work? Or could it be that the list I am trying to save is a empty list even if I use the same name? This is the event for the button where I register the loan.
private void btnRegister_Click(object sender, EventArgs e)
{
Kund lender = null;
Bok bookToLend = null;//Search for chosen customer foreach (Kund K in CustomerList) { //Om rätt kund hittas if (txtPersonNr.Text.Equals(K.PersonNr.ToString())) lender = K; //Kunden som hittats läggs i ny variabel. } //If customer wasn't found. if (lender == null) { textBox7.Text = "Kund saknas!"; return; } //Search for chosen book. foreach (Bok B in BookList) { //Om rätt bok hittas if (textBox6.Text.Equals(B.ISBN.ToString())) bookToLend = B; } //if book wasn´t found. if (bookToLend == null) { textBox7.Text = "Bok saknas!"; return; } if (bookToLend.BiblioteksKund == null) //If book doesn´t have its customer object set. { bookToLend.BiblioteksKund = lender; //Sets the customerobject of the book. lender.Loan.Add(bookToLend); //Places the book to lend in the customers booklist. textBox7.Text = "Lån registrerat!"; } }larsp777 wrote:
Or could it be that the list I am trying to save is a empty list even if I use the same name?
Looks that way; a Kund holds a list of books. When you serialize the Kund, you write the PersonNr and the Name - but not the list. Again, I'd recommend not saving a list of books, but numbers that are linked to a book.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
Or could it be that the list I am trying to save is a empty list even if I use the same name?
Looks that way; a Kund holds a list of books. When you serialize the Kund, you write the PersonNr and the Name - but not the list. Again, I'd recommend not saving a list of books, but numbers that are linked to a book.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Eddy Vluggen wrote:
Again, I'd recommend not saving a list of books, but numbers that are linked to a book.
Ok, Think I missunderstood you some. But I still have to save the objects somehow so I'm not really sure how that solves anything. Please explain. Edit: Realised that you probably ment the list saved in customer, not the list that holds all the books. That could be a way of avoiding cross-reference I guess.
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Eddy Vluggen wrote:
Again, I'd recommend not saving a list of books, but numbers that are linked to a book.
Ok, Think I missunderstood you some. But I still have to save the objects somehow so I'm not really sure how that solves anything. Please explain. Edit: Realised that you probably ment the list saved in customer, not the list that holds all the books. That could be a way of avoiding cross-reference I guess.
larsp777 wrote:
But I still have to save the objects somehow so I'm not really sure how that solves anything. Please explain.
It'd be a translation from a database-design. Right now, each book would be stored in a Kund? That means that you'd be "moving" the book-data when the book moves between different Kunds; it'd be saved in a different list. If you had three lists, life could become easier; one list to hold a collection of Kunds, one to hold the Books, and one to hold a pointer to Kund/Book combinations. In SQL, it'd be something like below;
CREATE TABLE Customer
(
Id INT IDENTITY(1,1)
PRIMARY KEY (Id)
)CREATE TABLE Book
(
Id INT IDENTITY(1,1)
PRIMARY KEY (Id)
)CREATE TABLE LentItems
(
CustomerId INT,
BookId INT
FOREIGN KEY (CustomerId) REFERENCES Book(Id),
FOREIGN KEY (BookId) REFERENCES Book(Id)
)Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
But I still have to save the objects somehow so I'm not really sure how that solves anything. Please explain.
It'd be a translation from a database-design. Right now, each book would be stored in a Kund? That means that you'd be "moving" the book-data when the book moves between different Kunds; it'd be saved in a different list. If you had three lists, life could become easier; one list to hold a collection of Kunds, one to hold the Books, and one to hold a pointer to Kund/Book combinations. In SQL, it'd be something like below;
CREATE TABLE Customer
(
Id INT IDENTITY(1,1)
PRIMARY KEY (Id)
)CREATE TABLE Book
(
Id INT IDENTITY(1,1)
PRIMARY KEY (Id)
)CREATE TABLE LentItems
(
CustomerId INT,
BookId INT
FOREIGN KEY (CustomerId) REFERENCES Book(Id),
FOREIGN KEY (BookId) REFERENCES Book(Id)
)Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Eddy Vluggen wrote:
Right now, each book would be stored in a Kund? That means that you'd be "moving" the book-data when the book moves between different Kunds; it'd be saved in a different list.
This is how it works (as far as I know): Every book is stored in a list BookList and every customer is saved in CustomerList; Every customer (Kund) has it´s own list loan where references are saved to books that are borrowed by that customer. Again, this was a project I made in Java for a University-course a while back. Been trying to transfer it to C#. Every book (Bok in Swedish) has a reference to the customer who borrowed the book, so it "knows" who borrowed it. The savingpart worked fine in Java but maybe works differently in C#.
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Eddy Vluggen wrote:
Right now, each book would be stored in a Kund? That means that you'd be "moving" the book-data when the book moves between different Kunds; it'd be saved in a different list.
This is how it works (as far as I know): Every book is stored in a list BookList and every customer is saved in CustomerList; Every customer (Kund) has it´s own list loan where references are saved to books that are borrowed by that customer. Again, this was a project I made in Java for a University-course a while back. Been trying to transfer it to C#. Every book (Bok in Swedish) has a reference to the customer who borrowed the book, so it "knows" who borrowed it. The savingpart worked fine in Java but maybe works differently in C#.
larsp777 wrote:
Again, this was a project I made in Java for a University-course a while back. Been trying to transfer it to C#.
Every book (Bok in Swedish) has a reference to the customer who borrowed the book, so it "knows" who borrowed it. The savingpart worked fine in Java but maybe works differently in C#.Those references are pointers; I don't think that the XmlSerializer is going to save the private loan-list on it's own.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
Again, this was a project I made in Java for a University-course a while back. Been trying to transfer it to C#.
Every book (Bok in Swedish) has a reference to the customer who borrowed the book, so it "knows" who borrowed it. The savingpart worked fine in Java but maybe works differently in C#.Those references are pointers; I don't think that the XmlSerializer is going to save the private loan-list on it's own.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
Eddy Vluggen wrote:
Those references are pointers; I don't think that the XmlSerializer is going to save the private loan-list on it's own.
Yes, I know they are pointers. That is why you get cross-reference, isn´t it? Still, you could be right in that this is the problem. Edit: I think this was one thing I considered when I was trying to find the answer. Saving a list of books was no problem.
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Eddy Vluggen wrote:
Those references are pointers; I don't think that the XmlSerializer is going to save the private loan-list on it's own.
Yes, I know they are pointers. That is why you get cross-reference, isn´t it? Still, you could be right in that this is the problem. Edit: I think this was one thing I considered when I was trying to find the answer. Saving a list of books was no problem.
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larsp777 wrote:
No, I don't think I did it in XML. I saved the objects in a binary format.
:) If you save a collection using the binary-formatter, then it'll also save the pointers. If you use XML, you'll have to save the relations yourself, or keep a list.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]
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larsp777 wrote:
No, I don't think I did it in XML. I saved the objects in a binary format.
:) If you save a collection using the binary-formatter, then it'll also save the pointers. If you use XML, you'll have to save the relations yourself, or keep a list.
Bastard Programmer from Hell :suss: If you can't read my code, try converting it here[^]