Difference between revisions of "Internet Transfer Control in VB6"

Latest revision as of 11:41, 24 November 2010

This control is good for short pages only. Microsoft's Internet Transfer control has a bug whereby you will not always get the full contents of the page. See the WinInet section of this page for details on the work around. There is a fix out by Microsoft in the form of a service pack. If you use the service pack fix then you won't need to concern yourself with the bug or work around.

• Visual Studio Service Pack 6

Inet Transfer Control / ITC

The Internet Transfer ActiveX Control component can be used to download files from the Internet. ITC can be used to send and retrieve documents across the net. ITC supports both synchronous and asynchronous transfers. In synchronous transfer, you can make the system wait until the transfer is complete, and in asynchronous transfer, the system can work as usual while the file is transferred in the background.

Properties

AccessType property

• icUseDefault
• icDirect
• icNamedProxy

protocols

• icUnknown
• icDefault
• icFTP
• icGopher
• icHTTP
• icHTTPS

icHTTP and icFTP

• URL - The complete URL of what you want to communicate with. e.g. http://wiki.robotz.com/
• RemoteHost - The name/IP address of the remote host you are connecting with.
• RemotePort - The port to connect on. For HTTP connections, you want port 80. For FTP, port 21.
• Document - The name of an individual document
• UserName - Username to login with
• Password - Password that goes with the username

Inet Control States

The Internet Transfer Control provides us with one event, StateChanged. And, when this event is raised, it provides only one parameters, State. We can use this parameter to detect what state the control is in, and then react accordingly.

State constants:

• icConnected (4) - Connected
• icConnecting (3) - Connecting
• icDisconnected (10) - Disconnected
• icDisconnecting (9) - Disconnecting
• icError (11) - An error has occurred
• icHostResolved (2) - The remote address given has been accepted
• icNone (0) - No current state
• icReceivingResponse (7) - A response is current being received
• icRequesting (5) - Data is currently being requested
• icRequestSent (6) - A data request has been sent
• icResolvingHost (1) - The remote address given is currently being checked
• icResponseCompleted (12) - A response was successfully completed
• icResponseReceived (8) - A response has been received

in order by state number (same as above):

'  0 ........................ stateless
'  1 .......... checking remote address
'  2 .......... remote address accepted
'  3 ....................... connecting
'  4 ........................ connected
'  5 .................. data requesting
'  6 ..................... request sent
'  7 ............... response receiving
'  8 ................ response received
'  9 .................... disconnecting
' 10 ..................... disconnected 
' 11 ............................ error
' 12 .......................... success

Here is a State to Text Function you may find useful:

Private Function StateText(n As Integer) As String
  Select Case n
    Case 0:    StateText = "stateless"
    Case 1:    StateText = "checking remote address"
    Case 2:    StateText = "remote address accepted"
    Case 3:    StateText = "connecting"
    Case 4:    StateText = "connected"
    Case 5:    StateText = "data requesting"
    Case 6:    StateText = "data sent"
    Case 7:    StateText = "response receiving"
    Case 8:    StateText = "response received"
    Case 9:    StateText = "disconnecting"
    Case 10:    StateText = "disconnected"
    Case 11:    StateText = "error"
    Case 12:    StateText = "success"
    Case Else:    StateText = "that's weird"
  End Select
End Function

icConnected provides you with a failsafe way of knowing that you are connected to a remote machine. As icDisconnected also provides you with a way of knowing when you are offline. Probably the most useful constant is icError. This tells us that an error has occurred.

ErrorConstants:

• icBadUrl
• icCannotConnect
• icConnectFailed
• icConnectionAborted
• icNoRemoteHost

On Error Goto vbErrHand

vbErrHand:
Msgbox Err.Description

There is no way to detect runtime errors.

Using the OpenURL Method to Retrieve an HTML Page

Example 1: VB6

 Private Sub cmdGo_Click()
   inet.Protocol = icHTTP
   inet.URL = txtAddress.Text
   txtOut.Text = inet.OpenURL
 
 End Sub
 
 Private Sub Form_Load()
   txtAddress.Text = "http://wiki.robotz.com"
 End Sub

Example 2: Using byte array then converting

Private Sub cmdGo_Click()
  Dim bAr() As Byte, cnt As Integer
  inet.Protocol = icHTTP
  inet.URL = txtAddress.Text
  bAr() = inet.OpenURL(, icByteArray)
  txtOut.Text = StrConv(bAr, vbUnicode)
End Sub

Example 3: VBA, Uses the internet transfer control with HTTP to create an HTML page.

 Dim objInet as Inet
 
 Private Sub Form_Load()
    ' Set a reference to the internet transfer control.
    Set objInet = Me!axInetTran.Object
 End Sub
 
 Private Sub cmdWriteFile_Click()
    Dim b() as Byte
 
    ' Set the internet transfer control protocol and URL.
    objInet.Protocol = icHTTP
    objInet.URL = "HTTP://www.microsoft.com"
 
    ' Retrieve the HTML data into a byte array.
    b() = objInet.OpenURL(objInet.URL,icByteArray)
 
    ' Create a local file from the retrieved data.
    Open "C:\Homepage.htm" For Binary Access Write As #1
    Put #1, , b()
    Close #1
 
    MsgBox "Done"
 End Sub
 
 Private Sub cmdGetHeader_Click()
    ' Set the internet transfer control protocol and URL.
    objInet.Protocol = icHTTP
    objInet.URL = "HTTP://www.microsoft.com"
 
    ' Open the HTML and display the header information.
    objInet.openURL objInet.URL, icByteArray
    MsgBox objInet.GetHeader
 End Sub

FTP Files Using the Internet Transfer Control

You can easily FTP files between a client and host computer.

Example 3: Working example to Retrieve a File from an FTP Site

   strExe = "GET " & strSrc & " " & strDst
   With inet
     .Protocol = icFTP
     .UserName = "username"
     .Password = "password"
     .RemoteHost = "ftp.robotz.com"
     .Execute , strExe
   End With

You can tell the program to continue while waiting for the connection to finish.

'Essential to avoid tying up the system
Do Until inet.StillExecuting = False ' WAIT Downloading..
  DoEvents
Loop

WinInet API Alternative (work around)

When you use the OpenUrl method of Microsoft Internet Transfer Control (ITC) to download files from Web servers, the resulting files may not be complete if the server uses "chunked" transfer-encoding to send the HTTP response data. ITC allocates a buffer of 64K bytes and calls InternetReadFile in a loop. Whenever the buffer containing the read data is not full after the API returns, ITC will exit the loop. However, InternetReadFile can return when less than the total number of bytes requested were read into the buffer, which causes the problem. This is the case if the transfer-encoding is chunked.

Ref: http://support.microsoft.com/kb/q232194/

Note: The bug was corrected in a service pack for Visual Studio 6.0. You may use the service pack rather than this work around below.

A way to work around this problem is to call the WinInet APIs directly in Visual Basic, as described in the following two steps:

Work Around Step 1

 Option Explicit
 
 Public Const INTERNET_OPEN_TYPE_PRECONFIG = 0
 Public Const INTERNET_OPEN_TYPE_DIRECT = 1
 Public Const INTERNET_OPEN_TYPE_PROXY = 3
 
 Public Const scUserAgent = "VB OpenUrl"
 Public Const INTERNET_FLAG_RELOAD = &H80000000
 
 Public Declare Function InternetOpen Lib "wininet.dll" Alias "InternetOpenA" _
 (ByVal sAgent As String, ByVal lAccessType As Long, ByVal sProxyName As String, _
 ByVal sProxyBypass As String, ByVal lFlags As Long) As Long
 
 Public Declare Function InternetOpenUrl Lib "wininet.dll" Alias "InternetOpenUrlA" _
 (ByVal hOpen As Long, ByVal sUrl As String, ByVal sHeaders As String, _
 ByVal lLength As Long, ByVal lFlags As Long, ByVal lContext As Long) As Long
 
 Public Declare Function InternetReadFile Lib "wininet.dll" _
 (ByVal hFile As Long, ByVal sBuffer As String, ByVal lNumBytesToRead As Long, _
 lNumberOfBytesRead As Long) As Integer
 
 Public Declare Function InternetCloseHandle Lib "wininet.dll" _
 (ByVal hInet As Long) As Integer

Work Around Step 2

 Private Sub Command1_Click()
     Dim hOpen               As Long
     Dim hOpenUrl            As Long
     Dim sUrl                As String
     Dim bDoLoop             As Boolean
     Dim bRet                As Boolean
     Dim sReadBuffer         As String * 2048
     Dim lNumberOfBytesRead  As Long
     Dim sBuffer             As String
 
     sUrl = "http://www.microsoft.com"
     
     hOpen = InternetOpen(scUserAgent, INTERNET_OPEN_TYPE_PRECONFIG, vbNullString, vbNullString, 0)    
     hOpenUrl = InternetOpenUrl(hOpen, sUrl, vbNullString, 0, INTERNET_FLAG_RELOAD, 0)
 
     bDoLoop = True
     While bDoLoop        
         sReadBuffer = vbNullString
         bRet = InternetReadFile(hOpenUrl, sReadBuffer, Len(sReadBuffer), lNumberOfBytesRead)
         sBuffer = sBuffer & Left$(sReadBuffer, lNumberOfBytesRead)
         If Not CBool(lNumberOfBytesRead) Then bDoLoop = False
     Wend
     
     Open "C:\Temp\log.txt" For Binary Access Write As #1
     Put #1, , sBuffer
     Close #1
     
     If hOpenUrl <> 0 Then InternetCloseHandle (hOpenUrl)
     If hOpen <> 0 Then InternetCloseHandle (hOpen)
 End Sub