generic list copy

[aku 10]

i am doing this
public class Books
{
public List<Book> listbook = null;
public Books()
{
listbook = new List<Book>();
}
public void insert(Book p)
{
listbook.Add(p);
}
}
class query<Book> : Books
{
public List<Book> execute()
{
List<Book> newlist = listbook.ToList();
return newlist;
}
}
it gives error
Error 1 Cannot implicitly convert type 'System.Collections.Generic.List<gen_code.Book>' to 'System.Collections.Generic.List<Book>' C:\Users\nasir87us\AppData\Local\Temporary Projects\PAQL\gen_code.cs 77 32 PAQL

[Joseph Martell]

I believe that your problem is hidden by the fact that you have used the name "Book" in your generic class declaration.

This declaration:

class query<Book> : Books
{
    public List<Book> execute()
    {
        List<Book> newlist = listbook.ToList();
        return newlist;
    }
}
 

creates a generic class using the name "Book" as a place holder for whatever class someone uses when they declare an instance of your query class. It does NOT mean that your generic class is using the Book class. In your class definition, you could replace "Book" with "T" and the following would be the result:

class query<T> : Books
{
    public List<T> execute()
    {
        List<T> newlist = listbook.ToList();
        return newlist;
    }
}
 

The error is saying that this line:

List<T> newlist = listbook.ToList();
 

is causing a conversion error because there is no defined conversion between listbook.ToList() - which returns a List<Book> - to a List<T> which is the declared type of newlist.

I think your ultimate solution is to define your query class as NOT generic. It doesn't seem to be important to the functionality of your class as you have it written here especially since you are inheriting the Books class which explicitly uses the Book class. Is there some reason why this would not work?

class query : Books
{
    public List<Book> execute()
    {
        //you don't need the newlist variable either, btw
        return listbook.ToList();
    }
}
 

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