Paint event problem

[chriswallis]

I am struggling to understand when the paint event should
fire, since I have created a very simply app to simply
draw a line on a picturebox whenever the paint event
fires. Unfortunately the line only appears when I move
another window over the picturebox, not when the form is
restored from being minimized or focus is switched to
another window and back again. How can I ensure that the
line is always painted?

The app is very simply. Create a form with a picturebox
called picMain and add the following code.

Private Sub Form1_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load
DrawLine()
End Sub

Private Sub DrawLine()

Dim g As Graphics
Dim oPen As New Pen(Color.Black)

Try
g = picMain.CreateGraphics
g.DrawLine(oPen, 0, 0, 200, 200)
Finally
oPen.Dispose()
oPen = Nothing
g.Dispose()
g = Nothing
End Try
End Sub

Private Sub picMain_Paint(ByVal sender As Object,
ByVal e As System.Windows.Forms.PaintEventArgs) Handles
picMain.Paint
DrawLine()
End Sub

 

[krisbrixon]

(Let me prefix this statment with that I never have used the drawing thing before)

I know this does not solve your problem, but I copied the code over and I tried it many different ways. I works fine if you based it off the form and not the picture box. I was having the same problems as you, but one thing I noticed was if you resize the form by grabbing the edges of the form the line will show up (note: I anchored the picturebox on all four sides). Otherwise the line will show up and disappear (very quickly).

Not that this solves the problem but I found a shorter way to write the code.

Private oPen As New Pen(Color.Black)
...
Private Sub picMain_Paint(ByVal sender As Object, ByVal e As System.Windows.Forms.PaintEventArgs) Handles MyBase.Paint
picMain.CreateGraphics.DrawLine(oPen, 0, 0, 200, 200)
End Sub

 

[krisbrixon]

I found another person with the same problem. It does not list a solution, but it does have his email address. Send him a line and see if he solved the problem.

http://www.codehound.com/groups/def...95bc.0204230602.476204f4@posting.google.com_5

Entire Thread
Author: Jason (oldfriend99@hotmail.com)
Subject: vb.net drawing in a picturebox?
Newsgroup: microsoft.public.vb.controls
Date: 04/23/2002 at 7:02 AM

.....

 

[chriswallis]

I have solved this myself. The trick is to pass in the Graphics object to the DrawLine function from the paint event. ie:

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
DrawLine(picMain.CreateGraphics)
End Sub

Private Sub DrawLine(g as Graphics)

Dim oPen As New Pen(Color.Black)

Try
g.DrawLine(oPen, 0, 0, 200, 200)
Finally
oPen.Dispose()
oPen = Nothing
g.Dispose()
g = Nothing
End Try
End Sub

Private Sub picMain_Paint(ByVal sender As Object,
ByVal e As System.Windows.Forms.PaintEventArgs) Handles
picMain.Paint
DrawLine(e.graphics)
End Sub

I guess this is all to do with when DeviceContexts are created and destroyed for the picturebox...