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Using WINAPI calls to download a very large file. Fatal error occurs only when I try to do a DoEvents. Need to update a progressBar, as the file downloads.. (VarPtr might be issue)

Dear Experts

Ok, I've got this program, and I'm updating it to .net. The portion of the program with the error is supposed to retreive a file over the internet, and save it to disk. This worked in vb6, and I'm not sure why it's not working here. I feel that it is /very/ close to working, but it isn't quite right. I think there is something wrong with the way I'm trying to compensate for the now non-existant VarPtr function. Anyways, two things are happening. The first, the file just starts at 0k and stays there, and the second is this fatal error which occurs whenever I try to put in any line relating to a 'DoEvents' call. I think when my program does the doEvents, it is losing the handles somehow, or windows is moving them or something. Here is the code for this, any help would be greatly appreciated!

~Michael@iondataexpress.com

'---------------- solution off of experts exchange
      'https://www.experts-exchange.com/questions/22084328/Download-Progress.html

      Private Declare Function InternetOpenW Lib "wininet.dll" (ByVal lpszCallerName As Integer, ByVal dwAccessType As Integer, ByVal lpszProxyName As Integer, ByVal lpszProxyBypass As Integer, ByVal dwFlags As Integer) As Integer
      Private Declare Function InternetOpenUrlW Lib "wininet.dll" (ByVal hInternet As Integer, ByVal lpszUrl As Integer, ByVal lpszHeaders As Integer, ByVal dwHeadersLength As Integer, ByVal dwFlags As Integer, ByVal dwContext As Integer) As Integer
      Private Declare Function InternetReadFile Lib "wininet.dll" (ByVal hFile As Integer, ByVal sBuffer As Integer, ByVal lNumBytesToRead As Integer, ByRef lNumberOfBytesRead As Integer) As Integer
      Private Declare Function InternetCloseHandle Lib "wininet.dll" (ByVal hInternet As Integer) As Integer
      Private Declare Function CreateFileW Lib "kernel32" (ByVal lpFileName As Integer, ByVal dwDesiredAccess As Integer, ByVal dwShareMode As Integer, ByVal lpSecurityAttributes As Integer, ByVal dwCreationDisposition As Integer, ByVal dwFlagsAndAttributes As Integer, ByVal hTemplateFile As Integer) As Integer
      Private Declare Function WriteFile Lib "kernel32" (ByVal hFile As Integer, ByVal lpBuffer As Integer, ByVal nNumberOfBytesToWrite As Integer, ByRef lpNumberOfBytesWritten As Integer, ByVal lpOverlapped As Integer) As Integer
      Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Integer) As Integer

      Const GWrite As Integer = 1073741824
      Const Create As Integer = 2
      Const Context As Integer = 2
      Const Multiple As Integer = 1024
      Const Reload As Integer = &H80000000

'if the other varPtr doesn't work, use this one
Public Function VarPtr(ByVal o As Object) As Integer
Dim GC As System.Runtime.InteropServices.GCHandle = System.Runtime.InteropServices.GCHandle.Alloc(o, System.Runtime.InteropServices.GCHandleType.Pinned)
Dim ret As Integer = GC.AddrOfPinnedObject.ToInt32
GC.Free()
Return ret
End Function

Public Sub httpGet(ByVal File As String, ByVal httpUrl As String, Optional ByRef Kilobytes As Integer = 1)
Dim bytes() As Byte
Dim hOpen As Integer
Dim hConn As Integer
Dim cFile As Integer
Dim bSize As Integer
Dim bRead As Integer
Dim myFile As Scripting.File

Dim handle As Runtime.InteropServices.GCHandle
Dim strPtr As IntPtr

'Dim value As String = "Bob"
'handle = Runtime.InteropServices.GCHandle.Alloc(value, Runtime.InteropServices.GCHandleType.Pinned)
''2) Get the address of the pinned object
'Dim strPtr As IntPtr = handle.AddrOfPinnedObject()
''3) Free the handle:
'handle.Free()

hOpen = InternetOpenW(0, 1, 0, 0, 1)

handle = Runtime.InteropServices.GCHandle.Alloc(httpUrl, Runtime.InteropServices.GCHandleType.Pinned)
strPtr = handle.AddrOfPinnedObject()
hConn = InternetOpenUrlW(hOpen, strPtr, 0, 0, Reload, Context)
handle.Free()

handle = Runtime.InteropServices.GCHandle.Alloc(File, Runtime.InteropServices.GCHandleType.Pinned)
strPtr = handle.AddrOfPinnedObject
cFile = CreateFileW(strPtr, GWrite, 0, 0, Create, 0, 0)
handle.Free()

myFile = mainForm.myFSO.GetFile(File)

If cFile <> (-1) And hConn <> 0 Then
bSize = (Kilobytes * Multiple)
ReDim bytes(bSize)
Do
'UPGRADE_ISSUE: VarPtr function is not supported. Click for more: 'mshelp://MS.VSCC.v80/dv_commoner/local/redirect.htm?keyword="367764E5-F3F8-4E43-AC3E-7FE0B5E074E2"'
If InternetReadFile(hConn, VarPtr(bytes(0)), bSize, bRead) Then
'UPGRADE_ISSUE: VarPtr function is not supported. Click for more: 'ms-help://MS.VSCC.v80/dv_commoner/local/redirect.htm?keyword="367764E5-F3F8-4E43-AC3E-7FE0B5E074E2"'
WriteFile(cFile, VarPtr(bytes(0)), bSize, 0, 0)
Else
Exit Do
End If
'Application.DoEvents() <<<<------This is the line that errs out.
frmProgress.Refresh()
If frmProgress.ProgressBar1.Maximum >= myFile.Size Then frmProgress.ProgressBar1.Value = myFile.Size
Loop While bRead <> 0
End If
InternetCloseHandle(hConn)
InternetCloseHandle(hOpen)
CloseHandle(cFile)
End Sub

These are the errors and descriptions for this (interestingly enough, each time I run it, it doesn't always produce the same error:

1) AccessViolationException: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

2) FatalExecutionEngineError was detected: The runtime has encountered a fatal error. The address of the error was at 0x7a00d227, on thread 0xc94. The error code is 0xc0000005. This error may be a bug in the CLR or in the unsafe or non-verifiable portions of user code. Common sources of this bug include user marshaling errors for COM-interop or PInvoke, which may corrupt the stack.

shahprabal

I haven't looked closely at your code but seems like declaring a new thread to update the progressbar should be the way to go...

Jeanette Durham

ASKER

How would you declare a new thread?

ASKER CERTIFIED SOLUTION

shahprabal

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Jeanette Durham

ASKER

Alright, thanks shaprabal, I was able to eventually fix it by creating a new thread, as you suggested. This is my working code, for completion. Maybe someday someone else will need a really good download routine..

Private Declare Function URLDownloadToFile Lib "urlmon" Alias "URLDownloadToFileA"(ByVal pCaller As Integer, ByVal szURL As String, ByVal szFileName As String, ByVal dwReserved As Integer, ByVal lpfnCB As Integer) As Integer

Dim myThread As System.Threading.Thread 'This object will allow us to download the file in a seperate thread

Public Sub GetFile(ByRef filePath As String, ByRef fileURL As String)
Dim llRetVal As Integer ', filePath$, fileURL$
llRetVal = URLDownloadToFile(0, fileURL, filePath, 0, 0)
End Sub

Public Sub DoMeter()
Dim myFile As Scripting.File
On Error Resume Next
Do
myFile = mainForm.myFSO.GetFile(cDNCFilePathEXE)
Loop Until Not myFile Is Nothing
On Error GoTo 0
'frmProgress.aut()
Do
' frmProgress.Refresh()
frmProgress.ProgressBar1.Value = myFile.Size
Application.DoEvents()
Loop Until myFile.Size >= gDNCFileSize
frmProgress.lblDescription.Text = "Finished downloading file!"
End Sub

Public Sub GetDNCDBForReal()
GetFile(cDNCFilePathEXE, cFileURL)
'frmProgress.Visible = True
'myThreadMeter.Start()
'Do
'frmProgress.ProgressBar1.Refresh()
'Loop Until frmProgress.ProgressBar1.Value >= gDNCFileSize
'Stop
BootAppAndWait(cDNCFilePathEXE)
End Sub

Public Sub downloadDoNotCallDB()
If modFiles.doesNeedUpdate() = True Then
'If gDNCFileSize = 0 Then Exit Sub
'filePath = App.Path & "\DNC.exe"
frmProgress.Show()
myFunctions.centerForm(frmProgress)
frmProgress.ProgressBar1.Maximum = gDNCFileSize
frmProgress.Text = "Downloading 'DNC.exe'.."
frmProgress.lblDescription.Text = "Downloading latest 'Do Not Call' Database." & vbCrLf & "Saving file to " & cDNCFilePath

myThread = New System.Threading.Thread(AddressOf GetDNCDBForReal)
'myThreadMeter = New System.Threading.Thread(AddressOf DoMeter)

'myThread.IsBackground = True
myThread.Start()
DoMeter()

'modFiles.GetFile cDNCFilePathEXE, cFileURL
'httpGet(cDNCFilePathEXE, cFileURL)
'Dim myFile As Scripting.File
'On Error Resume Next
'Do
' myFile = mainForm.myFSO.GetFile(cDNCFilePathEXE)
'Loop Until Not myFile Is Nothing
'On Error GoTo 0

Else
MsgBox("Your `Do Not Call Database' is already up to date. Try again in a few days.")
End If
frmProgress.btnClose.Enabled = True
mainForm.checkForDNCDB()
End Sub