A small tutorial for the beginner on how to handle objects.
So, you're working in VB2005 or VB2008 (or any object oriented language) and would like to know a little more about why you sometimes get strange results...
What I mean by strange results? Well look at this code and run it:
CODE
Option Strict On
Option Explicit On
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim theFirstObject As New aSmallClass
Dim theSecondObject As New aSmallClass
theFirstObject.name = "Jens"
theSecondObject.name = "Jenny"
theSecondObject = theFirstObject
theFirstObject.name = "John"
MsgBox(theFirstObject.name)
MsgBox(theSecondObject.name)
Me.Close()
End Sub
End Class
Public Class aSmallClass
Public name As String
End Class
If you find the result obvious then this tutorial is not for you.
Still reading? Ok, what's happening here?
We declare two object variables and assign two separate objects to them. We do
theSecondObject = theFirstObject, thinking that we will make a copy of the first object and its data. Then we change what's inside theFirstObject and after that we show what's inside both objects. I was very confused by the result; both objects contain the string "John".
The reason for this confusion is that there is a misconception regarding what is happening above. To explain this I'll be a little lengthy but bear with me...
Suppose you have two signs, shaped like arrows and containing text, furthermore you have two houses. In the first house there's a guy named Jens living and in the second house there's a gal named Jenny living. The first sign points at Jens' house, the second at Jennys house. Suppose that you turn the second sign so it points at Jens' house and then Jens moves out and John moves in.
Now, ask yourself: If I look at the firs sign; whose house does this point at? Then ask yourself: If I look at the second sign; whose house does this point at? The result is - of course - both signs point at Johns house.
Here comes a leap of the mind!
Look at the code above while thinking of theFirstObject and theSecondObject as signs specialized in pointing at houses of the aSmallClass type! Can you see what's happening? We create two signs and two houses in the Dim-statements, then Jens moves in to the first house
theFirstObject.name = "Jens" and Jenny moves in to the second house
theSecondObject.name = "Jenny", after this we turn Jennys sign so it points at Jens house
theSecondObject = theFirstObject and by now Jens moves out and John moves in
theFirstObject.name = "John". After all this pointing, turning and moving we print out who lives in the house pointed out by the first and second sign respectively.
What have we learned?
The objects them self are containers for data (as a matter of speech at least) and the variables we associate with the objects are in fact pointers (signs) that only point out objects. This means that when we think we are working with objects we are in fact often working with pointers. This sometimes leads to confusion.
By now I hope that you will be less confused and have enough knowledge about objects and their associated variables to have an idea about what's wrong when you get weird results.
Ok?!! Do I have to care, besides this obscure example, what does it matter?
Suppose you have an object and you want to copy it together with all its content and associate it with a variable. Maybe because you want to update an object with new info and compare the original to what is stored in a database before updating the database. This to make sure that no one has updated the database from somewhere else while your user is updating her object. In this case you could make a copy of the original object and let the user update the copy. Before updating the database you compare the database with the original object and if they are the same it's ok to update, otherwise you'll have to inform your user that a change has occurred while she was working and inhibit the update, showing the user the new info.
So, there is a reason to copy an object with all it's content. How do we do this? Actually there are several methods and some mumbo jumbo talk pertaining to this but I'll use the above code - slightly modified - for a simple example of an unsofisticated method.
CODE
Option Strict On
Option Explicit On
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Dim theFirstObject As New aSmallClass
Dim theSecondObject As New aSmallClass
theFirstObject.name = "Jens"
theSecondObject.name = "Jenny"
theSecondObject = theFirstObject.CopyMe 'Changed here
theFirstObject.name = "John"
MsgBox(theFirstObject.name)
MsgBox(theSecondObject.name)
Me.Close()
End Sub
End Class
Public Class aSmallClass
Public name As String
Public Function CopyMe() As aSmallClass 'Added
Dim aNewSmallClass As New aSmallClass
aNewSmallClass.name = Me.name
Return aNewSmallClass
End Function
End Class
Notice how I create a new object of the aSmallClass type in the CopyMe function, then I copy the original objects data field (name) to the new object. Now I'm done creating an object and copying the original data to the new object so I return a variable of type aSmallClass to the outside world: The variable named theSecondObject is hereby "turned" and pointed to the new object that was returned by the CopyMe function.
After all this we have theFirstObject pointing at an object containing "Jens" and theSecondObject pointing at (hear hear!!) a completely different object - but - this second object is also populated by "Jens". On the next row we let John move in to the first object while Jens still lives in the second.
Since this pretty much concludes this tutorial I hope you now understand why the result is two message boxes, the first one displaying "John" and the second displaying "Jens".
A final word:
When talking to hard core programmers you might come across the expressions "deep copy" and "shallow copy", a slight knowledge of the difference may come in handy.
* A deep copy completely copies one object to another. See below; both the "identity" and the "innerClass" will be completely copied, change anything in the copy and the original will be unaffected.
* A shallow copy copies only data that are not objects (strings are, or may be, an exception), i.e. all variables. Stuff like integers are copied and variables associated to objects are copied but inner objects are not copied. See below: A shallow copy of an object of type aSmallClass will have its own identity, change it in the copy and the original is unaffected. However, both the original and the copy will have their innerClass variable pointing at the same object. Change something in the copys inner object and it will show in the original too.
CODE
Public Class aSmallClass
Public identity As Integer
Public innerClass as anotherSmallClass 'Variable for other class
End Class
Thanks for today class, ave a nice afternoon!
...but hey?!!! Wait! What happened to the first object that was pointed out by the theSecondObject variable, the one containing "Jenny"?
When we re-associate the variable theSecondObject from Jennys object to a copy of Jens' object there is no sign pointing out Jennys object any more. With no variable pointing to it we can not find it again. It is lost for ever in the voids of cyberspace
This leads on to the subject of memory management in VB. There is a thing called "garbage collector" (GC) that runs in the background whenever VB is executing a program. The GC roams the memory of the computer looking for abandoned objects. When it finds an object that has no associated variables (no signs pointing at it) it grimly kills the object and lets the previously occupied memory free for other use.
This really concludes my little tutorial. I hope I've helped at least one person to understand more than before. Thank you for reading.
Regards
/Jens
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Understanding object handling for beginners, Some object oddities explained for beginners.
12 Dec, 2008 - 01:18 PM
10 out of 10 for style and presentation 
...I'll be watching and waiting.
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