如何从另一个类中的另一个函数访问新的实例化类方法？

Question

实例化一个类，然后在相同的函数中使用它时我感到很舒服，但是现在我需要通过工具栏按钮实例化一个类：

Dim pll As New Polyline()
Debug.WriteLine("TSB_Polyline_Click:: a new polyLine is born : " & pll.Count)
pll.AddPoint(0, 0)

然后，我需要从另一个子类中的类方法运行pll.AddPoint：

Public Sub MyEvent(sender as object, e as myEvent) Handles myEvent
Dim x as Double, y as Double
pll.AddPoint(x,y)

有我的错误（System.NullReferenceException：'对象引用未设置为对象的实例。'），pll =没什么（我的构造函数中没有错误，我的类通过工具栏button_Click进行工作）

我试图将Pll声明为公开并共享：

Public Shared pll As Polyline

甚至是导入：

Imports oGIS.Polyline

没有任何作用。我的班级在第一个Sub（工具栏按钮）中得到实例化，但是以某种方式在离开Sub ...时就死了...

还有其他解决方案，使用VB6代替VB.Net吗？

我有点绝望，我在Google的任何地方都找不到这样的话题...

Answer 1

如果需要使用多个方法访问变量，则应在所有变量之外声明该变量。如果不需要在声明它的类型之外访问它，则应为Private；如果您特别需要该类的所有实例访问同一对象，则应为Shared。在您的情况下，Private而不是Shared是显而易见的选择：

Private pll As Polyline

您现在在第一种方法中设置了该字段，而在第二种方法中得到了。

Answer 2

我想对您的课程进行部分看会是这样...

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

声明一个类级别（在这种情况下，该类是一个Form）变量，您的表单中的任何方法都可以看到并使用它。

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