How do I clear a drawn line in vb.net? - vb.net

Question

I am writing a simple test program that draws an axis/crosshair in a form. I have two text boxes, where I put in the x-center and y-center and draw the crosshair based on that. I want to be able to put in new coordinates, and move the crosshair to the new position, but when I do, the old drawing stays there. I want to erase the old drawing and then draw the new one.
My code is below:
Public Class Form1
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim mypen As Pen
mypen = New Pen(Drawing.Color.Red, 1)
Dim mygraphics As Graphics = Me.CreateGraphics
Dim x_center = Integer.Parse(xPos.Text)
Dim y_center = Integer.Parse(yPos.Text)
mygraphics.DrawLine(mypen, x_center - 50, x_center, x_center + 50, x_center)
mygraphics.DrawLine(mypen, y_center, y_center - 50, y_center, y_center + 50)
End Sub
End Class

Answer 1

The Drawing on a Control surface is usually handled through the Control's Paint() event, using its PaintEventArgs class object.
To raise the Paint() event of a Control, call its Invalidate() method.
(Note that the Invalidate() method has a number of overloads, some of which allows to re-paint only a defined region of the surface.)
If a Graphics object is created elsewhere (as you're doing now), the drawings performed with this object will persist or will be erased when you don't want to (e.g. if a Control needs to repaint itself - and this happens quite often - the drawings will be erased).
Also, the Graphics object can't be stored. It will become an invalid object as soon as a Control has repainted its surface.
You could re-design you code in this way.
Create a shared Pen (you can redefined it at any moment if you need to, using its properties) so you don't have to create a new one every time you need to draw something.
Use a shared Point field to store the current center of the drawing.
Move the Graphics.DrawLine() to the Paint event of your Form.
Remember to Dispose() the Pen object when the Form closes (you can use it's Dispose() pre-defined method).
Public Class Form1
Private mypen As Pen = New Pen(Color.Red, 1)
Private Position As Point = New Point(-1, -1)
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
If (Integer.TryParse(xPos.Text, Position.X) = True) AndAlso
(Integer.TryParse(yPos.Text, Position.Y) = True) Then
Me.Invalidate()
End If
End Sub
Private Sub Form1_Paint(sender As Object, e As PaintEventArgs) Handles MyBase.Paint
If Position.X > -1 Then
e.Graphics.DrawLine(mypen, Position.X - 50, Position.Y, Position.X + 50, Position.Y)
e.Graphics.DrawLine(mypen, Position.X, Position.Y - 50, Position.X, Position.Y + 50)
End If
End Sub
End Class
This is, however, not that much efficient, because you need to invalidate the entire Form.
For a full implementation, take a look a this Class (PasteBin - CrossHair).