creating variable in a class

Ashmadia · April 28th, 2011, 10:33 PM

Hi

This may be beyond the scope of the book, but I'm working on an assignment for a visual basic class I'm taking.

The goal is to make a store front for a movie store.

I've been able to pass the database record key to a variable in the code-behind of my browse.aspx. When I pass the variable to the ShoppingCart class, I get a null reference exception. My code below isn't exact, but it's close to what I'm working on.

this works fine:

Code:

[browse.aspx button-click event]
Dim cart as New Shopping Cart = nothing
Dim dk as string
dk= detailsview.datakey.value.tostring()
cart.Add(dk)

Then in the ShoppingCart.vb file I have

Code:

Imports System.Collection

Public Class ShoppingCart
Dim cart as new stringcollection = nothing

Public Sub Add(ByVal value as string)
     cart.Add(value)
End Sub

The cart.Add throws a null reference exception and I cannot figure out why. On the other hand, I'm not even sure this is a good way to approach the problem, since once the user navigates to the cart.aspx I have no idea if I'll even be able to access the cart created on browse.aspx

Thanks =)

Imar · April 30th, 2011, 05:27 AM

Hi there,

Quote:

This may be beyond the scope of the book

Yes, it is ;-) You're much better off posting questions like these in a more general ASP.NET category: http://p2p.wrox.com/asp-net-4-539/ Makes it easier for everyone to find what they are looking for.

Anyway, you need to post your real code. There's not much point trying to debug code that doesn't even compile....

Cheers,

Imar

Niraj Singh · April 30th, 2011, 06:20 AM

Class variables means Object of Class,Because an instance of class can access the all instances and Method of class ,
Class sum
{
int a,b;
public static void main()
{
sum s=new sum() //This is a variable of class.......sum
s.a=20;
s.b=40;
}
}

Imar · April 30th, 2011, 07:45 AM

Hi Niraj Singh,

How does this answer the question or help the OP? His question is about a null reference exception in VB code, and as a reply you explain the concept of a class? And in C#?

Imar

Niraj Singh · April 30th, 2011, 01:52 PM

Class is an User Defined Data type.Class is also an object.............
Class has blue print for accessing/Creating Variable and Method.

Imar · April 30th, 2011, 02:06 PM

Yes, but what does that have to do with the original question? It's like replying and saying: an orange is a citrus fruit. True, but not very helpful.... :-)

And BTW: a class is not an object. You use a class to create objects (instances) of that class.

Cheers,

Imar