How to access a new instantiated class method from another function in another class? - vb.net

Question

I feel comfortable when instantiating a class, then using it within the same function, but now I need to instantiate a class from a toolbar button:
Dim pll As New Polyline()
Debug.WriteLine("TSB_Polyline_Click:: a new polyLine is born : " & pll.Count)
pll.AddPoint(0, 0)
And then I need to run the pll.AddPoint from my class method in another sub:
Public Sub MyEvent(sender as object, e as myEvent) Handles myEvent
Dim x as Double, y as Double
pll.AddPoint(x,y)
There I have my error (System.NullReferenceException: 'Object reference not set to an instance of an object.'), pll = Nothing (no error in my constructor, my class worked from the toolbar button_Click)
I tried to declare pll public and shared:
Public Shared pll As Polyline
And even to import it:
Imports oGIS.Polyline
Nothing works. My Class is instanciated in the first Sub (toolbar button), but somehow dies when leaving the Sub...
Is there any other solution that doing it using VB6 instead of VB.Net ?
I am a bit desesperate, I found no such topic anywhere on Google...

Answer 1

If you need to access a variable in more than one method then it should be declared outside all of them. It should be Private if you don't need to access it outside the type it's declared in and it should only be Shared if you specifically need all instances of the class accessing the same object. In your case, Private and not Shared is the obvious choice:
Private pll As Polyline
You now set that field in the first method and get it in the second.

Answer 2

I am guessing a partial look at your class would be something like this...
Public Class Polyline
Public ReadOnly Property CountOfPoints As Integer
Get
Return ListOfPoints.Count
End Get
End Property
Public Property ListOfPoints As New List(Of Point)
Public Sub AddPoint(x As Integer, y As Integer)
Dim p As New Point(x, y)
ListOfPoints.Add(p)
End Sub
End Class
Declare a class level (in this case the class is a Form) variable which can be seen and used by any method in your form.
Public Class Form1
Private LocalPolyLine As Polyline
'This is a local list, not part of the class
'It is a list of the number of instances of the class you have created
Private ListOfLines As New List(Of Polyline)
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Pretend this is a button on your toolbar used to create a new line
'Each time you click this button you have a completely new line with no points
LocalPolyLine = New Polyline
'If you want to keep track of how many lines you have...
ListOfLines.Add(LocalPolyLine)
End Sub
Private Sub AddPoint(x As Integer, y As Integer)
Debug.Print($"Number of Lines is {ListOfLines.Count}")
Debug.Print($"Number of points on current line {LocalPolyLine.CountOfPoints}")
LocalPolyLine.AddPoint(x, y)
Debug.Print($"Number of points on current line {LocalPolyLine.CountOfPoints}")
End Sub
'To see all the points on the line...
Private Sub ViewPoints()
For Each p In LocalPolyLine.ListOfPoints
Debug.Print($"The coordinates of the point are {p.X},{p.Y}")
Next
End Sub
End Class