Null Reference Exception for Class Lists

Question

I am new to programming and am running into an issue when creating a class with a list property of another class and then accessing it in main. I am getting the exception \"

Answer 1

When you create BookList, you haven't actually initialized the list that is its member. You can do this by changing your initialization to:

BookList myBookList = new BookList() {bookList = new List<Book>()};

Or by writing a constructor for the BookList class which initializes the list; which would look like this:

class BookList
{
    public List<Book> bookList { get; set; }

    public BookList(){ //New constructor
        bookList = new List<Book>();
    }
}

The reason you get this error is that while you've created an instance of BookList, you haven't actually make sure that the BookList's inner booklist property is initialized. It's like if you tried to do this:

List<string> newList;
newList.Add("foo");

That wouldn't work because you've only declared the newList, not initialized it.

Answer 2

Your problem is with public List<Book> bookList { get; set; } in your BookList class; you've never initialized it. You can do this, for example, from within the constructor:

class BookList
{
    public List<Book> bookList { get; set; }

    public BookList
    {
        bookList = new List<Book>();
    }
}

Please note: properties should always be named with Pascal casing, i.e. all letters uppercase: BookList not bookList.

Also, since it doesn't really make sense to replace your BookList with another instance of List<Book>, I would suggest you set the access modifier on the setter to private so it can only be initialized once, i.e. in the constructor:

public List<Book> bookList { get; private set; } // notice the "private" set

Answer 3

You need to initialize the Boolist.bookList in the default ctor:

class BookList
{
    public BookList() { bookList = new List<Book>; } // <<<<
    public List<Book> bookList { get; set; }
}