This tutorial will teach you how to use the AddHandler keyword in VB.NET

Intro
This tutorial might be a bit tricky for some (you might wonder what the point of this is).
Alright, let's get started.

Now go to form1_paint and type in
e.Graphics.DrawLine(New Pen(Color.Black), 0, 0, 10, 10)

Run it, you should see a line going from (0,0) to (10,10). Easy enough.

Create a new project and create a new class called GameClass.
Now, let's go to GameClass, and add a constructor:

Dim frm as Form
Public Sub New(GameForm as Form)
frm = GameForm
End Sub

Easy enough.

Now, when programming games, we usually don't want our "game code" in form1. We usually want some other class to handle that for us (in this case GameClass). So create a Gameclass.Paint event:

Public Sub Paint(e as PaintEventArgs)
 e.Graphics.DrawLine(New Pen(Color.Red), 20, 20, 30, 30)
End Sub

We want our GameClass to do the painting for us, and that's what we're doing.

In form1:
Dim game as New Gameclass(Me)

In form1_paint, add:
Game.Paint(e)

Now run and you should see two lines, a black one and a red one (in different places). Our goal is accomplished. Now our drawing code can be kept inside gameclass and we don't have to use form1's paint event.

AddHandler
However that's a waste of memory. A total waste of memory. Why? Ok, think about programming an actual game for a second. How many times are you going to call the paint event per second? Hundreds of times (depending on how fast the PC is of course). An average first person shooter (3d) gets 30 FPS (meaning that 30 frames are being rendered per second). Let's go with this 30 frames per second thing.  If we pass the e argument 30 times per second into GameClass - we're really wasting memory if you think about it. As a matter of fact it's a complete waste of memory.

How do we get around this? This is where AddHandler comes in. In form1_paint delete the line that says Game.Paint(e).

Now go to the constructor of GameClass and type in the following:

 AddHandler Gameform.Paint, AddressOf Me.Paint

<Ignore the error you get when you type this in>
This is basically saying, whenever the Paint event in the form is called, let the Paint event in this class be called too.

You might wonder what AddressOf does. The word Address here refers to the memory address. Just like you have a house address or an IP address or an email address, memory has an address too. When your program is compiled and when it runs, all the subs that you create are there somewhere on your RAM. This AddHandler statement doesn't implicitly type in ("behind the scenes") Game.Paint in the paint event of form1. What it does is much more efficient - it calls the gamecalss.paint event directly through memory using the memory address provided, making this a much faster operation. If you don't understand this, all you need to know is that this is much faster than calling the sub directly.

Now about that error.
Method 'Public Sub Paint(e as PaintEventArgs)' does not have the same signature as delegate 'Delegate Sub PaintEventHandler(sender As Object, e As System.Windows.Forms.PaintEventArgs)'.

Uhh, all you have to do is change the Paint sub's arguments.... like this:
Public Sub Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs)

It's doing this to make sure that the arguments match with the form1_Paint arguments.

Wrap Up
As I said before, you might wonder what the point of this is. The point of it is to speed up your application first of all.

Also, here are some usages for it. Besides being able to draw in GameClass, let's say you want GameClass to also handle key movement. Heh heh. There you go! All you have to do is AddHandler for that. As long as you have the Public Sub New(GameForm as Form) argument, you're fine - you can use any of the form's methods.

You'll find use for this in the RPG Programming Tutorial Series, especially when I do the Event-Driven Programming tutorial.

There is no source code for this tutorial as it is relatively short