Xor Pen mode in vb?

hi,
i have written the following code in which when the button is pressed first time ,a line is drawn,but when the button is pressed 2nd time the drawing mode becomes Xor Pen which is 7.after that the line is drawn at the same place as before.According to Xor mode if any thing drawn over the same thing,then that thing is erased and back pixels are shown.Please tell me that what is wrong with this code?, because it is not working.
thanx

Dim button As Integer

Private Sub Command1_Click()

If button = 1 Then
Form1.DrawMode = 7
Form1.Line (100, 100)-(800, 800)

End If

If button = 0 Then
Form1.Line (100, 100)-(800, 800)
button = 1
End If


End Sub
if i have done any thing wrong please tell me the mistake