Cannot access a disposed object on form close - vb.net

Question

I am trying to make some code for a game that has a grid and a tick counter but the grid just tries to fill the window instead of stopping and does not display the tick counter. The error that keeps coming up is:
A first chance exception of type 'System.ObjectDisposedException' occurred in System.Windows.Forms.dll
I have no idea what this means and I don't know how to fix it.
Heres my code:
Public Class Form1
Dim G As Graphics
Dim BBG As Graphics
Dim BB As Bitmap
Dim r As Rectangle
Dim tSec As Integer = TimeOfDay.Second
Dim tTicks As Integer = 0
Dim MaxTicks As Integer = 0
Dim IsRunning As Boolean = True
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Me.Show()
Me.Focus()
G = Me.CreateGraphics
BB = New Bitmap(Me.Width, Me.Height)
StartGameLoop()
End Sub
Private Sub DrawGraphics()
For X = 0 To 19
For Y = 0 To 14
r = New Rectangle(X * 32, Y * 32, 32, 32)
G.FillRectangle(Brushes.BurlyWood, r)
G.DrawRectangle(Pens.Black, r)
Next
Next
G.DrawString("Ticks: " & tTicks & vbCrLf & _
"TPS: " & MaxTicks, Me.Font, Brushes.Black, 650, 0)
G = Graphics.FromImage(BB)
BBG = Me.CreateGraphics
BBG.DrawImage(BB, 0, 0, Me.Width, Me.Height)
G.Clear(Color.Wheat)
End Sub
Private Sub StartGameLoop()
Do While IsRunning = True
Application.DoEvents()
DrawGraphics()
TickCounter()
Loop
End Sub
Private Sub TickCounter()
If tSec = TimeOfDay.Second And IsRunning = True Then
tTicks = tTicks + 1
Else
MaxTicks = tTicks
tTicks = 0
tSec = TimeOfDay.Second
End If
End Sub
End Class

Answer 1

You are utilizing a lot of bad practice here...
First of all, using Me.CreateGraphics() is bad and the resulting object can only be drawn with once, which means that you are forced to call it several times. Continuously calling it will just create more and more graphics objects that increases memory usage. Even if you were to dispose of them every time you've finished drawing it's still a huge bottleneck because it slows down processing.
Secondly, using Application.DoEvents() is VERY BAD PRACTICE and will burn your CPU in a loop like that. Unless used correctly (which you don't) it can cause unexpected and unpredictable behaviour. The error you're getting is a good example of such unexpected behaviour.
I suggest you read this MSDN blog, it explains exactly why one shouldn't use Application.DoEvents(): Keeping your UI Responsive and the Dangers of Application.DoEvents.
Instead, in order to do this correctly:
Replace Me.CreateGraphics() with your form's Paint event and do all the drawing in there through the e.Graphics object.
Replace your game loop with a Timer that continuously calls Me.Invalidate() to redraw the form.
For example:
Dim WithEvents GameTimer As New Timer() With {.Interval = 1}
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
GameTimer.Start()
End Sub
Private GameTimer_Tick(sender As System.Object, e As System.EventArgs) Handles GameTimer.Tick
Me.Invalidate() 'Redraw the form.
TickCounter()
End Sub
Private Sub Form1_Paint(sender As System.Object, e As System.Windows.Forms.PaintEventArgs) Handles Me.Paint
e.Graphics.Clear(Color.Wheat)
For X = 0 To 19
For Y = 0 To 14
Dim r As New Rectangle(X * 32, Y * 32, 32, 32)
e.Graphics.FillRectangle(Brushes.BurlyWood, r)
e.Graphics.DrawRectangle(Pens.Black, r)
Next
Next
e.Graphics.DrawString("Ticks: " & tTicks & vbCrLf & _
"TPS: " & MaxTicks, Me.Font, Brushes.Black, 650, 0)
End Sub