My code only works when I insert a random MsgBox - vb.net

Question

I've encountered a very bizarre problem when trying to get a screenshot of a TableLayoutPanel in my form.
I have this code (taken from another question (How to get a screenshot, only for a picturebox); code courtesy of user "Chase Rocker"):
Dim s As Size = TableLayoutPanel1.Size
Dim memoryImage = New Bitmap(s.Width, s.Height)
Dim memoryGraphics As Graphics = Graphics.FromImage(memoryImage)
Dim ScreenPos As Point = Me.TableLayoutPanel1.PointToScreen(New Point(0, 0))
memoryGraphics.CopyFromScreen(ScreenPos.X, ScreenPos.Y, 0, 0, s)
Form3.PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize
Form3.PictureBox1.BringToFront()
Form3.PictureBox1.Image = memoryImage
Now, here comes my problem. This code gives me a blank picture. It takes the screenshot apparently, but all I can see is white. Now, I was trying to see if the size was correct, so I was messing with MsgBox. I add this line to the code:
MsgBox("Random Message")
Getting
Dim s As Size = TableLayoutPanel1.Size
MsgBox("Random Message")
Dim memoryImage = New Bitmap(s.Width, s.Height)
Dim memoryGraphics As Graphics = Graphics.FromImage(memoryImage)
Dim ScreenPos As Point = Me.TableLayoutPanel1.PointToScreen(New Point(0, 0))
memoryGraphics.CopyFromScreen(ScreenPos.X, ScreenPos.Y, 0, 0, s)
Form3.PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize
Form3.PictureBox1.BringToFront()
Form3.PictureBox1.Image = memoryImage
By some reason I don't know, the screenshot now works. I don't see white anymore, but the actual screenshot of the TableLayoutPanel. For me is very weird that the code only works with a MsgBox. Maybe I'm missing something. Does anyone know why this happens? Thank you!

Answer 1

How about if you try to make the TableLayoutPanel draw itself to a bitmap instead? This can be done using the Control.DrawToBitmap() method.
Dim s As Size = TableLayoutPanel1.Size
Dim memoryImage As New Bitmap(s.Width, s.Height)
TableLayoutPanel1.DrawToBitmap(memoryImage, New Rectangle(New Point(0, 0), s))
Form3.PictureBox1.Image = memoryImage

Answer 2

If the TableLayoutPanel fill happens in the same event handler where you grab the image then Windows has not draw the UI for the elements added to the TableLayoutPanel. Only when you exit from the event handler, the winforms engine has the opportunity to draw everything.
Adding a MessageBox changes everything because calling Show (a modal call that interrupts your code and pass control back to window) allows the Winform engine to draw the pending updates and your code works.
You can add a Timer control and put the code that execute the ScreenShoot in the Timer event.
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
......
' code that fills the TableLayoutPanel
......
Dim tim1 = new System.Windows.Forms.Timer()
tim1.Interval = 1
AddHandler tim1.Tick, AddressOf tim1Ticked
tim1.Start()
End Sub
Private Sub tim1Ticked(sender As Object, e As EventArgs)
......
' Code that execute the screenshoot.
......
Dim t = DirectCast(sender, System.Windows.Forms.Timer)
t.Stop()
End Sub