Internet - File Transfers

In the Professional and Enterprise Editions of VB are the two controls which provide Internet features - the Internet Transfer Control and the Web Browser Control. The Internet Transfer Control (ITC) provides both FTP and HTTP file transfer controls and is the subject of this tutorial - as is the use of the wininet.dll, which also provides file transfer capabilities that you can access from within your VB applications.

The wininet.dll file is simply a library of functions that you can use to do file transfers. It is not a part of VB6, but is installed along with the Microsoft Internet Explorer. This means you can safely assume that it is available on most PCs.

The reason for using the ITC is simplicity. The control provides a very straightforward, fairly simple interface to use in your VB programs. The down side is that the control is about 126K in size, increasing the size of the installation files your application users will have to download.

By using the existing wininet.dll file, you eliminate the increased application distribution file size, plus the wininet.dll offers greater control over the file transfer process. As you would expect, the wininet.dll is harder to learn and is not documented as part of the VB documentation package.

So, I'll focus first on the ITC, then end with enough detail on wininet.dll to show you how to use it as an alternative to the ITC.

Protocols

For the purposes of file transfer as discussed in this tutorial, It's not really that important to understand the details of either protocol, but there are a few facts which are important to know. The key point to remember right now is that the code for using the ITC for an FTP or for an HTTP file transfer are slightly different.

Most web sites are accessible by both FTP and HTTP (i.e., servers typically run both FTP and HTTP server software for accessing content), so you can usually chose which approach to take.

In general, it really makes little difference which protocol you use. Many of the books I've read recommend the FTP protocol because it is more flexible than HTTP (i.e., read that as FTP has more features which can be controlled by code than does HTTP). I generally agree with that recommendation, but will confess that my own freeware applications now have an online update feature which is based on the HTTP protocol.

Here's the tradeoff that drove me to that decision:

• FTP - Many ISPs do not allow anonymous (i.e., no username and no password) FTP connections to a website. But, I do not want to put my username/password into a distributed application for fear of compromising security on my web site. I could password protect and expose just a particular directory on the web site, but I've chosen to take no risks in that area.
• HTTP - The ITC code for an HTTP file transfer is extremly simple (the FTP code is not that complicated, it's just that the HTTP code is simpler).

One of the key drawbacks of using the ITC for file transfer (regardless of the protocol that is used) is that it does not provide any built-in capability to identify how many bytes of the transfer are complete at any point in time. All you can tell from within your VB program is whether the file transfer is in progress, or that it has stopped (because of a successful transfer or some error that stopped the file transfer process). This is one of the key reasons you might be interested in looking at one of the 3rd party file transfer OCXs.

OpenURL Method of File Transfer

Now that I've gotten the introductory comments out of the way, let's talk about the details of the ITC. The two most important things to know about the ITC is that there are two methods of downloading files from a web site - the OpenURL method and the Execute method. Both support the FTP and HTTP file transfer protocols.

The OpenURL method is very simple. You put in a file name to download and tell the program whether the file is all text or binary. The code looks for an HTTP transfer of a text file looks like this:

text1.text = inet1.OpenURL ("http://www.vbinformation.com/badclick.htm", icString)

Dim bData() as Byte
bData() = inet1.OpenURL ("http://www.vbinformation.com/badclick.htm", icByteArray)

Since all files (text or binary) can be transferred as a binary file, I used the same file name in both examples. Note that in the first case, the downloaded file content is placed in a textbox named 'text1'. In the second case, the downloaded file content is saved in a Byte array whose upper bound is set by the number of bytes downloaded by the OpenURL method. Also, note that both examples use HTTP URLs, but FTP URLs could have been used just as readily.

In case you don't remember, an easy way to save the bData byte array is:

Open "filename" for Binary as #1
Put #1, , bData()
Close #1

This is really all there is to successfully downloading a file by using the OpenURL method. I'll cover the question of errors (such as when the server is down, or the file is not there) later in this tutorial.

You should note that the OpenURL method is synchronous - which simply means that any code that follows the OpenURL statement will not be executed until the file transfer is completed, or until the file transfer is stopped by the occurence of an error or by a user command (I'll show how to do this later).

Execute Method of File Transfer

The second method for downloading a file is the Execute method. As you'll see it provides more features, but is definitely more complicated to code. The one key difference that you'll want to be aware of is that with the Execute method the bytes of data are sometimes, but not always, kept within the ITC itself (in a memory buffer). When the ITC does keep the downloaded bytes in its buffer, you must use another method called GetChunk to extract the dowloaded bytes. Whether the memory buffer is used varies with the arguments used in calling the Execute method. I'll give more detail on that later.

Another key difference that you should know about is the Execute method is asynchronous - meaning that it will download the file in the background and that any code following the Execute statement will be executed immediately.

Finally, to complicate the discussion a bit more, the arguments you use in the Execute method differ depending on whether you want to use FTP or HTTP for the file transfer.

Here's the general syntax for the Execute method:

inet1.Execute (url, operation, data, requestheaders)

Here's an example of the code you would use to start a transfer using the Execute method and the FTP protocol:

inet1.Execute ("ftp://www.microsoft.com", "DIR")

The Execute method actually supports 14 FTP commands (which are placed in the 'operation' argument), but there are primarily three (CD, GET, and PUT) which you will use most often:

• inet1.Execute ("ftp://www.microsoft.com", "CD newdirectory"
• inet1.Execute ("ftp://www.microsoft.com", "GET remotefile localfile"
• inet1.Execute ("ftp://www.microsoft.com", "PUT localfile remotefile"

Also, you will note that the GET and PUT commands create a file on either the local or remote computers. In these cases, the ITC memory buffer is not used. However, the ITC memory buffer must be accessed in order to get the output of the 'DIR' command.

In order to discuss how the ITC memory buffer is accessed, we have to talk first about the StateChanged Event. Statechanged is the only event the ITC control has, and it provides a variable called 'State' which must be read to determine the status of a pending Execute method.

The State values are:

• icNone (0)
• icHostResolvingHost (1)
• icHostResolved (2)
• icConnecting (3)
• icConnected (4)
• icRequesting (5)
• icRequestSent (6)
• icReceivingResponse (7)
• icResponseReceived (8)
• icDisconnecting (9)
• icDisconnected (10)
• icError (11)
• icResponseCompleted (12)

Typically, Select Case code is used within the StateChanged event to determine what action to take. In general, only actions 8, 11, and 12 are used to generate a code response. The others are used mostly to decide what message to display in a status label/toolbar.

For State=12, where the file transfer is complete, the action you take is entirely up to you. This would usually be a simple popup message telling the user that the file transfer is complete.

For State=11, which indicates that an error has occurred, you would have to generate code necessary to correct or ignore the error condition.

Generally, you simply wait for State=12 to indicate that the transfer is complete. But, in some cases you may want to begin extracting data before the transfer is complete. For example, the HTTP header information is received first, but is not included in the ITC download buffer. To get that information you use the .GetHeader method. You can use the State=8 to determine when the header information is available.

In those cases where the ITC buffer is used to temporarily store the downloaded information, the .GetChunk method is used. Here's the code for the case where string data is being downloaded and a State=12 has been received to indicate that the transfer is complete:

Do
   DoEvents
   bData = inet1.GetChunk (1024, icString)
   AllData = AllData & bData
Loop Until bData = ""

Finally, remember that you may have to set the .UserName and .Password properties if you are using the FTP commands within the Execute method. If the FTP site you are accessing allows 'anonymous' logon's, then you will not have to set these properties.

Properties/Methods/Events

Properties	Methods	Events
AccessType	Cancel	StateChanged
Document	Execute
hInternet	GetChunk
Password	GetHeader
Protocol	OpenURL
Proxy
RemoteHost
RequestTimeout
ResponseCode
ResponseInfo
StillExcuting
URL
UserName

This table can be digested more easily if you think in terms of how the properties/methods/events are used. Here's the way I've grouped them in my notes:

• Basic
  The .OpenURL and .Execute methods are the heart of using the ITC. Everything you do requires the use of one of these two methods. The .GetChunk method is used to capture data downloaded by the .Execute method.
• Working
  The .URL, .Cancel, .StillExecuting, .ResponseCode, .ResponseInfo, .Cancel, .GetChunk, and .GetHeader interface elements are used extensively during program execution.
• Startup
  The .AccessType, .Proxy, .Protocol, .RequestTimeout, .RemoteHost, .UserName, .Password, and .RemotePort are very basic properties which you set once or use the default - then you're done with them.

As a final note, in case you've been watching carefully you will notice that I left out any discussion on the use of the .Execute method with HTTP commands. This was strictly for lack of time/space in this tutorial. The .Execute method can be used equally with either FTP or HTTP commands, but the FTP options are generally more extensive so FTP is the normal choice for programmers.

WinInet.dll - File Transfer Alternative