I am trying to get myself out of the dark ages and convert our Windows Desktop Apps to the web (VB.2015).

I have received some code from a developer to do this and I am not sure that it is correct for multi-users.

Given the below code. It appears to me that the AddInvoice being shared the variables both in the parameters and the inline definitions would not be unique since the class has not

Button event calls the class method fileImporter.readIt(
Code:
Public Class Importer
    Inherits BasePage
...
Protected Sub UploadBtn_Click(sender As Object, e As EventArgs)
 If Not fileReader.readIt(strPath, dsData, pOutReason) Then
                    messagesList.Add("File Name : " + FileUpLoad1.FileName.ToLower() + pOutReason) : GoTo ExitFunction
                Else
                    fileImporter.readIt(strPath, profile.OAuthAccessToken, profile.OAuthAccessTokenSecret, profile.RealmId, dsData, messagesList, "Read IIF file")
                End If
End Sub
End Class
But the class is never instantiated but just called so a new copy of the class is not created and thus only one set of variables in the machine.

Code:
Public Class FileImpoter(worksheetName As String, OAuthAccessToken As String, OAuthAccessTokenSecret As String, realmId As String, dsData As DataSet, ByRef messagesList As List(Of String), Optional ByRef pOutReason As String = "") As Boolean
...
Public Function readIt(...)
...
AddInvoice(OAuthAccessToken, OAuthAccessTokenSecret, realmId, messagesList, "Add Invoice", dsData)
...
End Function
...
Private Shared Function AddInvoice(...)
 Dim FunRC As Boolean = False
        Dim Ern As Integer
        Dim Erd As String
If ds1.Tables(0) IsNot Nothing AndAlso ds1.Tables(0).Rows.Count > 0 Then
For i = 0 To ds1.Tables(0).Rows.Count - 1
Dim row = ds1.Tables(0).Rows(i)
...
Next 
endif
...
End Function
End Class
So is the code incorrect? or do I have more to learn???