VB6 - How to detect a file is finished copying from an external source - vb6

Question

My software (written in VB6) needs to import csv files that can be large. Users are using copy/paste to place the files in the input folder.
How can I be sure the files I want to read are fully copied before processing them?
Things I tried :
Compare GetFileSizeString over a span of seconds : doesn't work, I get the final value even if the file has just begun to copy.
FileSystemObject.DateLastModified : same
FileSystemObject.DateLastAccessed : same
FileLen : same
FileDateTime : same
EDIT - Added Samba/Linux Info (from comments):
I'm having a hard time dealing with is from Samba/Linux. I don't know why, but when the file is copied by Samba, the read only attribute doesn't matter.

Answer 1

I've used this method which uses the API to test for exclusive access. I've never tried it on a platform other than Windows so, I guess result may vary. I use this in a module of frequently used methods, but I believe I have included the API calls, types, and constants used.
Private Const ERROR_SHARING_VIOLATION = 32&
Private Const GENERIC_WRITE = &H40000000
Private Const INVALID_HANDLE_VALUE = -1
Private Const OPEN_EXISTING = 3
Private Type SECURITY_ATTRIBUTES
nLength As Long
lpSecurityDescriptor As Long
bInheritHandle As Long
End Type
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Private Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, lpSecurityAttributes As SECURITY_ATTRIBUTES, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, ByVal hTemplateFile As Any) As Long
Public Function CanOpenExclusive(ByVal vFileName As String) As Boolean
Dim lngResult As Long
Dim udtSA As SECURITY_ATTRIBUTES
On Error GoTo errCanOpenExclusive
If Len(vFileName) > 0 Then
udtSA.nLength = Len(udtSA)
udtSA.bInheritHandle = 1&
udtSA.lpSecurityDescriptor = 0&
lngResult = CreateFile(vFileName, GENERIC_WRITE, 0&, udtSA, OPEN_EXISTING, 0&, 0&)
If lngResult <> INVALID_HANDLE_VALUE Then
Call CloseHandle(lngResult)
CanOpenExclusive = True
Else
Select Case Err.LastDllError 'some errors may indicate the file exists, but there was an error opening it
Case Is = ERROR_SHARING_VIOLATION
CanOpenExclusive = False
Case Else
GoTo errCanOpenExclusive
End Select
End If
End If
Exit Function
errCanOpenExclusive:
Err.Raise Err.Number, Err.Source & ":CanOpenExclusive", Err.Description
End Function

Answer 2

I would use the FileDateTime function to work with to calculate when a date/time value indicating the date and time that a file was created or last modified.
You can read up more on the file system usage HERE.
In the syntax above, pathname is a string expression specifying a valid path (it may optionally include the drive); drive is a string expression specifying a drive letter; and filespec, oldfilespec, and newfilespec are string expressions that specify a file (they may optionally include the drive and path).
Following is a set of functions that can be used with files (all are functions except SetAttr, which is a statement):
Function
Description
Syntax
FileDateTime
Returns a date/time value indicating the date and time that a file was created or last modified.
FileDateTime(filespec)
GetAttr
Returns an integer representing the attributes of a file, directory, or folder
GetAttr(filespec)
SetAttr
Statement that lets you specify the attributes for a file
SetAttr(filespec, attributes)
CurDir$ (or CurDir)
Returns a string that indicates the current path for a specified disk drive. In the syntax on the right, drivename is a string expression that specifies a valid disk drive designation
CurDir$(drivename)
Dir$ (or DIr)
Returns a string that indicates a file or directory matching specified conditions
Dir$(filespec [,attributes])
FileLen
Returns a Long specifying the length of a file in bytes. If the specified file is open when the FileLen function is called, the value returned represents the size of the file immediately before it was opened.
FileLen(pathname)
LOF
Returns a Long representing the size, in bytes, of a file opened using the Open statement.
FileLen(filenumber)