Add objects to session - asp.net

Question

I need some clarity about session and how to add objects, because I think I do it the wrong way.
First I create a session to hold a list of Products:
Session["ShoppingCart"] = new List<Products>();
To add Products to the list, I do like this:
Session["ShoppingCart"] = new Products { ID = productId, Name = name };
I guess this isn't the right way?

Answer 1

I guess this isn't the right way?
Yes, this isn't the right way (please skip towards the last paragraph of my answer to know the correct way - which is not to use ASP.NET session at all). The correct way is to first get the object you stored inside the session by trying it to cast it to the same .NET type yo uhave stored inside the session:
var products = Session["ShoppingCart"] as List<Products>;
and then if this item is not null add the corresponding product to the list. We should of course make the necessary type check that the session actually contained a value with the specified key and that this value is of the expected type:
if (products != null)
{
var product = new Products { ID = productId, Name = name };
products.Add(product);
}
Of course we are using object references here which will only work if you are storing your session in-memory (sessionState mode = InProc) which of course is absolutely a terrible disaster and something you should never do in production. In a production environment you are probably persisting your session in a session server or even SQL server, aren't you? In this case it is more than obvious that working with object references is a recipe for disaster. So in this case once you have added the new product to the session you should of course set back the new list value to the session which will serialize the object instance to the corresponding data store:
if (products != null)
{
var product = new Products { ID = productId, Name = name };
products.Add(product);
Session["ShoppingCart"] = products;
}
Now, after all this being said I must admit that using ASP.NET Session is probably the huge mistake you would ever commit in a real world application. So basically every time you are using Session["xxx"] you are doing it wrong. Simply search the entire solution for the Session keyword and just get rid of it.

Answer 2

In order to add itens to an existing list on the Session, you must first retrieve the list then add the object to it. Here's an example:
Session["ShoppingCart"] = new List<Products>();
List<Products> productsList = (List<Products>)Session["ShoppingCart"];
productsList.add(new Products { ID = productId, Name = name });
Session["ShoppingCart"] = productsList;