Beginning ASP.NET 4.5 in CSharp and VB Opsylum

588 x CHAPTER 15 WORKING WITH DATA — ADVANCED TOPICS
The code checks to make sure review is not Nothing/null before it tries to access its properties.
The chances of the review being null in this example are pretty small because you access the
Add/Edit page by clicking an existing item in the Reviews page, so you can be pretty sure the item
is there. However, this is not always the case, especially not in public-facing pages. Your clients
may have a bookmark for a page with a specific ID in the query string. If you then delete that item
from the database and your users access the page using the old bookmark, the review can’t be
found and a so-called Null Reference exception occurs.
The same defensive coding mechanism is used to preselect the genre in the drop-down list. In
this case you can be sure the Genre still exists in the database because there’s a relationship
between the Id column of the Genre table and the GenreId of the Review table. However, checking
to make sure an item exists in a DropDownList control before you try to select it is a best
practice and helps in avoiding other Null Reference exceptions. Because the DropDownList with
genres hasn’t been populated at this stage, you need to call DataBind() first. This forces the
EntityDataSource control to get the genres and add them to the DropDownList. Afterward, the
code can successfully find and preselect the appropriate item.
Finally, when you click the Save button for an edited item, the exact same code is fired that was
used to insert a new item.
If you were using validation controls (and you really should, as you learned in Chapter 9),
you need to check whether or not the page is valid before you proceed with saving the Review
instance:
VB.NET
Protected Sub SaveButton_Click(sender As Object, e As EventArgs) _
Handles SaveButton.Click
If (Page.IsValid) Then
Using myEntities As New PlanetWroxEntities()
C#
protected void SaveButton_Click(object sender, EventArgs e)
{
if (Page.IsValid)
{
using (PlanetWroxEntities myEntities = new PlanetWroxEntities())
This is really all there is to adding and editing new reviews using your own code against the Entities
Framework. I realize it may look a little funky at first because you need to reset your head, forget
about smart controls and their many properties and events, and think in straight code. However, EF
makes this pretty straightforward, and most of it comes down to querying entities, copying values
from or to an object’s properties, and calling SaveChanges to propagate the changes back to the
database.
Clearly, this is just the beginning. There’s a whole lot more you can do once you start writing your own
code, whether or not it targets EF. For more information, get yourself a copy of Programming Entity
Framework Second Edition by Julia Lerman or Professional ASP.NET 4.5 in C# and VB from Wrox
(ISBN: 978-1-118-31182-0).