Attempting to create shopping cart remove button - asp.net

Question

Note: This code actually codes from a tutorial book I'm reading at the moment, so you can imagine this is frustrating me greatly! I'm building a basic shopping cart, using VB.NET 4, Visual Studio 2010 and MS SQL Server R2 2008. The code below is supposed to remove items from the cart, by reading a session variable, editing it to remove the appropriate ProductID and return the amending string to the session variable. It is then supposed to rebind the data to the gridview (gvCart) to refresh the data...but it seems here there is an error.
Every time I build the site in Visual Studio, it validates fine. But every time I run the site, and attempt to use the remove button it gives me the error:
Incorrect syntax near ')'.
with the IDE pointing me toward the final parenthesis.
I have been pulling my hair out at this for a good 4 hours now, any advice appreciated!
Protected Sub gvCart_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles gvCart.SelectedIndexChanged
'This method is hooked to the Remove button & is removing items from the cart
Dim strProductId As String = gvCart.SelectedRow.Cells(1).Text
If Session("Cart") IsNot Nothing Then
'Remove selected ProductID from Session string and retrieve session string
Dim strCart As String = Session("Cart").ToString()
Dim arIDs As String() = strCart.Split(",")
'Iterate through ID's in the 'Cart' array and rebuild the string leaving out the selected ID
strCart = String.Empty
For Each str As String In arIDs
'use Trim to remove leading and trailing spaces
If str.Trim() <> strProductId.Trim() Then
strCart += str + ", "
End If
Next
'Remove trailing space and comma
If strCart.Length > 1 Then
strCart = strCart.Trim()
strCart = strCart.Substring(0, strCart.Length - 1)
End If
'Put back into session var
Session("Cart") = strCart
'Rebind gvCart, which forces the sqldatasource to requery
gvCart.DataBind()
End If
End Sub
[EDIT]
I am also including the code that runs for the event sqlCart_Selecting for completion sake:
Protected Sub sqlCart_Selecting(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.SqlDataSourceSelectingEventArgs) Handles sqlCart.Selecting
Trace.Warn("sqlCart_Selecting") 'aids debugging
Dim strCart As String = String.Empty
If Session("Cart") IsNot Nothing Then
strCart = Session("Cart").ToString
e.Command.CommandText &= " WHERE product.ProductID IN (" + strCart + ") AND culture.CultureID = 'en'"
Else
e.Cancel = True
End If
End Sub
[EDIT]
Also including the SQL query used by the gridview 'gvCart' and the code for displaying the carts contents on another page
<asp:SqlDataSource ID="sqlCart" runat="server" ConnectionString="<%$ ConnectionStrings:AdventureWorksConnectionString %>" SelectCommand="SELECT product.ProductID, product.Name, product.ProductNumber, product.Color, subcat.Name AS SubcategoryName, cat.Name AS CategoryName, description.Description FROM Production.Product product JOIN Production.ProductSubcategory subcat ON product.ProductSubcategoryID = subcat.ProductSubcategoryID JOIN Production.ProductCategory cat ON subcat.ProductCategoryID = cat.ProductCategoryID JOIN Production.ProductModel model on product.ProductModelID = model.ProductModelID JOIN Production.ProductModelProductDescriptionCulture culture ON model.ProductModelID = culture.ProductModelID JOIN Production.ProductDescription description ON culture.ProductDescriptionID = description.ProductDescriptionID"></asp:SqlDataSource>
Protected Sub btnAddToCart_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnAddToCart.Click
'The contents of the cart will be saved in a Session object as a string of coma-delimited values of ProductID's
Dim strCart As String = String.Empty
Dim strProductID As String = gvProducts.SelectedDataKey.Value.ToString()
If Session("Cart") Is Nothing Then
strCart = strProductID
Else
strCart = Session("Cart").ToString() + ", " + strProductID
End If
Session("Cart") = strCart
lblCart.Text = strCart
End Sub

Answer 1

I will take a shot here, since you are not showing us everything (the SQL query for instance).
But it feels like the SQL query is using IN clause. And it appears to be failing when the cart goes empty.
Since Session("Cart") contains the comma separated values of the product IDs, an empty list would make the following SQL query fail:
select id, name
from products
where id in ()
with the message:
Incorrect syntax near ')'.
You have to show us the portion of code that's reloading the query from Session("Cart"). The query should be reassembled.
But there's one trick you can use! If your product ids are numeric and always greater than zero, change this line:
strCart = String.Empty
to this:
strCart = '-1, '
Update
I will really teach how to fish on this one. :) the problem is here:
Dim strCart As String = String.Empty
If Session("Cart") IsNot Nothing Then
strCart = Session("Cart").ToString
e.Command.CommandText &= " WHERE product.ProductID IN (" + strCart + ") AND culture.CultureID = 'en'"
Else
e.Cancel = True
End If
When cart is empty, Session("Cart") is not Nothing but it's an empty string (provided that you removed the -1 workaround). If Session("Cart") is an empty string the where clause should be " WHERE culture.CultureID = 'en'" only, no mention to product ids. God speed!