Add objects from database to a List, using LINQ to SQL

Question

I have a table books and i want to put all of its contents into a List<book> (right now i'am stuck at adding a single element). I tried

public class DataContext
{
    LINQtoSQLDataContext db = new LINQtoSQLDataContext();
    List<book> books = new List<book>();
    public List<book> addBook()
    {
        books.Add((book)from book in db.books where book.bookID == 1 select book);
        return books;
    }
}

But when i try to set it as the source of my wpf dataGrid, it throws an error:

Unable to cast object of type 'System.Data.Linq.DataQuery`1[DataLayer.book]' to type 'DataLayer.book'.

So the question is, how do i properly convert data from the database to a list?

'Where' will return an array. You only want to add 1 item.

sr28
– sr28

2022-05-20 13:33:49 +00:00
Commented May 20, 2022 at 13:33 — sr28
– sr28, Commented May 20, 2022 at 13:33

Yong Shun · Accepted Answer · 2022-05-20 13:50:25Z

1

Just return the List of book as below:

public List<book> addBook()
{
    return (from book in db.books 
        where book.bookID == 1 
        select book)
        .ToList();
}

And it's weird to name the method addBook as the method's outcome is to return a list of book.

Name it as: public List<book> GetBooks()

While List<T>.Add(T) Method only allow to add single item to list, to add multiple items into list, you need List<T>.AddRange(IEnumerable<T>) Method.

edited May 20, 2022 at 13:50

answered May 20, 2022 at 13:40

Yong Shun

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1 Comment

Feldmarshall Over a year ago

You're completely right about that method name. I was just testing how these things work, and forgot to change. I won't edit the post however, as it would invalidate some of the replies.

Vinceenzo · Accepted Answer · 2022-05-20 13:49:08Z

1

if I understand your problem correctly, the solution is simpler than expected, try making a where condition like this:

public class DataContext
{
LINQtoSQLDataContext db = new LINQtoSQLDataContext();
List<book> books = new List<book>();
public List<book> addBook()
{
    books = db.book.Where(x => x.bookId == 1).ToList();
    return books;
} 
}

answered May 20, 2022 at 13:49

Vinceenzo

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3 Comments

Roe Over a year ago

This doesnt add the book, it overwrites any existing books in the list

Vinceenzo Over a year ago

Yes, I misunderstood, I saw that you have solved it so good continuation!

Roe Over a year ago

Well as it seems the question was wrong and your answer is what OP was looking for. so my bad!

Roe · Accepted Answer · 2022-05-20 14:00:18Z

1

Just as sr28 pointed out, this will return an array. You could either add the array to the books list. I personally prefer using the method syntax.

I don't really understand why you would add the book to this list. But here you go. Since you already defined books you dont need to return it, you can simply add book to books.

public void AddBook()
{
    var book = db.books.FirstOrDefault(b => b.BookId == 1);
    books.add(book);
}

edited May 20, 2022 at 14:00

answered May 20, 2022 at 13:53

Roe

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