Visual Basic

public class Main_Class
public _CO as Double
    Public Property CO() As Double
        Get
            Return _CO
        End Get
        Set(ByVal value As Double)
            _CO = value
        End Set
    End Property
End Class

public class SubClass
    Public ReadOnly Property CO1() As Double
        Get
            Dim uc As New Main_Class
            Return uc._CO
        End Get
    End Property
End Class

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        
        SerialPort1.Open()

    End Sub

Imports System.Threading

Public Class Form2

    Dim thr As Thread

    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Debug.Print(Form1.SerialPort1.IsOpen)
        thr = New Thread(AddressOf thrsub)
        thr.IsBackground = True
        thr.Start()

    End Sub



    Public Sub thrsub()
        Debug.Print(Form1.SerialPort1.IsOpen)
    End Sub


End Class