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Introduction
This article explains how to create a server component using Internet
Transfer Control (ITC). This component can be used to download files
from the Internet, especially image files. Some download components
allow you to download text or HTML files, but this component allows
you to download binary files. Created in Visual Basic (VB), it has
been tested in both VB and ASP. Images can be downloaded from a local
intranet as well as from remote sites. Today's applications are
expected to extend their reach globally. An application should be
able to download files from any computer connected to the Internet.
Luckily, Microsoft has created an ActiveX control that makes
accessing files on the Internet much easier.
click here to download the sample code used in this article
Internet Transfer Control
Internet Transfer Control can be used to send and retrieve documents
across the net. This control uses the File Transfer Protocol (FTP)
and Hypertext Transfer Protocol (HTTP). Using FTP, the user can send
and retrieve files from any system connected to the Internet. Users
can perform actions such as rename and delete files, create and
remove directories, etc. ITC is a very handy control for VB
programmers. Basically, the Internet is used for transferring data,
and ITC helps programmers to use the Internet in a better way. All of
the functionality of ITC is implemented through its methods, and it
requires no interface.
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. ITC has many
methods and properties that will not be discussed here in detail (
refer to MSDN
http://support.microsoft.com/support/kb/articles/Q163/6/53.asp
for complete documentation).
This article shows how to create and test this component in steps
that will make it easier to understand the functionality. The article
discusses methods and properties used in the component.
Step 1: Creating the project
Start a new "ActiveX DLL" project. Assign a valid name to the
project, such as "HTTP, " which is used here. Assign a name to the
class module, such as "Dload."
Step 2: Adding a form
Add a form to the current ActiveX project. This step is necessary in
order to place the Internet Transfer Control on a form. Through this
form, we will access the control's methods and properties. I have
done this to make programming simpler and to make it easier for
beginners to understand and use the control. Remember, we could have
accessed the control directly in the DLL project and accessed its
methods, but for simplicity, I have included a form to place the ITC
on.
Step 3: Adding functionality
First, we are going to add some properties to the class module. The
class builder utility can be used to add the following properties to
the class:
Private mvarStatus As Variant 'local copy
Private mvarProxyAddress As Variant 'local copy
Private mvarRemotePort1 As Variant 'local copy
Private mvarRemoteHost1 As Variant 'local copy
Private mvarProtocol1 As Variant 'local copy
Let's discuss these properties one by one.
The first property will be used to display the status of the file
transfer. The following messages are displayed:
- URL not provided
This message is displayed if the correct URL is not provided or is
not provided at all.
- Timed Out
This message is displayed when the time out occurs.
- Server not found
If the computer from where the file is to be downloaded is not found,
then this error message is displayed.
- File not found
If the file is not found on the remote computer, then this message
will tell the user that the file could not be found.
- File was downloaded successfully
This message is displayed if the file is successfully retrieved and
downloaded to the system.
The second property is used to get proxy address from the user. ITC
uses this property to set or get the name of the proxy server used to
access the Internet.
The third property will be used to connect to the remote port of the
server. ITC uses this property to set or get the port number of the
remote system to which the control connects.
The fourth property is used to access the remote host. ITC uses this
property to set or get the address of the remote system to which the
control sends and/or receives data.
The fifth property is used to set the protocol that we are going to
use in this component. As already discussed, there are two protocols
used in the ITC. We shall use the HTTP protocol in our component.
Now let's move on to the main function. Declare the
function as below:
Public Function DLoad(ByVal strURL As String, Optional ByVal
sOutPutFile As String)
Two parameters are passed to the function. The first one is the
string containing the address of the file to be downloaded, and the
second parameter is the string containing the name and path of the
output file to be generated. A file-handling routine is not included
in this component. This will have to be incorporated in the ASP
script or the VB application, whichever is used as a test driver.
Declare the variables that will be used throughout the class module,
as follows:
Dim bytes() As Byte
Dim fn As Integer
Dim msg As String
Dim buf As String
Dim fnf As Integer
Dim snf As Integer
Dim tout As Integer
Dim OK As Integer
Dim tempbuf As String
Dim strResult As String
Bytes() is the array of bytes that will contain the downloaded material.
fn will contain the number of the file that will be opened to be written to.
buf is a buffer that will contain the data.
fnf is an acronym for "file not found" and will contain the number
of files not found.
snf stands for "server not found" and will contain the number of
remote addresses not found.
tout stands for "timed out" and will contain the number of timed out messages.
"strResult" will contain the downloaded string.
Set the properties to default values if no values are provided by the user.
'Set the transfer protocol
If Not Protocol1 = "" Then
Form1.Inet1.Protocol = icHTTP 'CStr(Protocol1)
ElseIf Protocol1 = "http" Or Protocol1 = "HTTP" Then
Form1.Inet1.Protocol = icHTTP
Else
Form1.Inet1.Protocol = icHTTP
End If
'Set the Access Type to Default
Form1.Inet1.AccessType = icUseDefault
'Set the proxy address
If Not ProxyAddress = "" Then
Form1.Inet1.Proxy = ProxyAddress
End If
If Not RemotePort1 = "" Then
Form1.Inet1.RemotePort = RemotePort1
End If
Notice the way we are accessing the control's properties. Set the
protocol to HTTP. The HTTP protocol is a stateless protocol.
Originally, HTTP was designed for the transmission of text, but it
also can be used to transfer binary files. Set the AccessType
property to default. AccessType specifies how the control will access
the Internet. The three possible settings for this property follow:
- icUseDefault
The control uses the access settings that are specified in the
Windows registry to access the Internet.
- icDirect
The control has a direct connection to the Internet.
- icNamedProxy
The control uses a proxy server. The proxy server must be specified
in the control's Proxy property.
Similarly, set the remotehost and remoteport properties that are
supplied by the user.
On Error Resume Next
If strURL = "" Then
Status = "URL not provided"
Exit Function
End If
Form1.Inet1.URL = strURL
bytes() = Form1.Inet1.OpenURL(, icByteArray)
strResult = Form1.Inet1.OpenURL(strURL, icString)
'Essential to avoid tying up the system
Do Until Form1.Inet1.StillExecuting = False ' WAIT Downloading..
DoEvents
Loop
We use the OpenURL method of the control to retrieve the content from
the remote file. The easiest way to use the Internet Transfer Control
is with the OpenURL method. Two parameters are passed to this method.
The first parameter is the URL of the file to be downloaded, and the
second parameter is the datatype, which tells the control about the
datatype of the content to be downloaded from the remote server. This
can be text or binary. If you want to download a file from an FTP
server, then the URL parameter should contain something like this:
ftp://ftp.server.com/test.htm
In case of HTTP, provide the URL in the format shown below:
http://www.server.com/image1.gif
The values that can be provided to the datatype parameter follow:
The URL property of the control is used to assign the URL of the file
to be downloaded to the control. The OpenURL method is used twice,
first to retrieve the actual data and then to retrieve the data as a
text string. This may be inefficient, but I did this for
demonstration purposes only. The data is retrieved as a text string
because the first 50 characters of the downloaded string contain the
error messages, if any.
Do Until Form1.Inet1.StillExecuting = False ' WAIT Downloading..
DoEvents
Loop
The above statement checks the status of the control, which is
important since we want the system to wait for the control to
complete the transfer. If "StillExecuting" is true, then the control
is still busy downloading the file.
tempbuf = bytes()
'If the URL returned anything, put the first
'50 characters in a buffer, Error messages
'will be found here.
If Len(strResult) > 50 Then
buf = Left(strResult, 100)
Else
buf = bytes()
End If
'Trap a time-out error
If Err = 35761 Then
tout = tout + 1
Status = "Timed out: " & tout
Err.Clear
Exit Function
'If nothing is returned, it means
'the server was not found.
ElseIf tempbuf = "" Then
snf = snf + 1
Status = "Server not found: " & snf
Exit Function
'If nothing is returned, it means
'the server was found, but the requested file was
'not present.
ElseIf InStr(1, buf, "404") Then
fnf = fnf + 1
Status = "File not found: " & fnf
Exit Function
Else
'Otherwise, everything is OK
OK = OK + 1
Status = "File was downloaded successfully: " & OK
End If
If the downloaded string is larger than 50 chars, then transfer the
first 100 characters to the buffer so that the error messages can be
searched for. "35761" is the time-out error.
'Get a file number
fn = FreeFile
'Open a binary file and load data into it!
Open sOutPutFile For Binary Access Write As #fn
Put #fn, , bytes()
DoEvents
'Close the open file
Close #fn
It's time to write the information retrieved from the remote server
to the local file. We have used the FreeFile function here. FreeFile
function returns an integer representing the next file number
available for use by the open statement. One to 255 file numbers can
be used to open files not accessible to other applications. Use file
numbers in the range of 256 - 511 for files accessible from other
applications. SOutPutFile is the name of the output file provided by
the user. We have opened the file for binary access. Put the
downloaded bytes into the newly opened file. After writing the
content to the file, close the file. That's it. I use this component
to download image files from remote computers, and it always works
fine.
Private Sub Class_Terminate()
Set Form1 = Nothing
End Sub
In the class_terminate() event of the class, destroy the form on
which we placed the control.
Step 4: Compile the project
To compile the project, from the file menu, select the "Make
HTTP.dll." The component is automatically registered on the computer
where it is compiled, but to register it on other computers, you will
have to register it manually. Use regsvr32.exe to register the
component manually.
Step 5: Testing the component
It's good practice to test the component in VB before taking it to
the ASP. If you find an error while using the component in ASP, it is
quite laborious to recompile the component with new changes added to
it. If such a situation occurs, stop the IIS services and unregister
the component before deleting it. To unregister the component, use
the following command:
Regsvr32.exe /u component.dll
Recompile the component, and start the IIS services. To test the
component in VB, add a standard EXE project to the current project.
Add two command buttons on the form, change the caption of one button
to "Test Dload Component," and change the caption of the other button
to "Exit," as follows:
Private Sub Test_Click()
Dim objHTTP As New HTTP.DLoad
Dim strURL As String
objHTTP.Protocol1 = icHTTP
objHTTP.ProxyAddress = ""
objHTTP.RemotePort1 = 80
strURL = "http://server/application/images/image1.gif"
objHTTP.DLoad strURL, "c:\test.gif"
MsgBox objHTTP.Status
Set objHTTP = Nothing
End Sub.
The test code is rather simple. Set the property values and provide
the name of the file to be downloaded.
Use the code below to test the component in ASP.
Dim objHTTP
Set objHTTP = server.createobject("HTTP.Dload")
Dim strURL
objHTTP.Protocol1 = icHTTP
objHTTP.ProxyAddress = ""
objHTTP.RemotePort1 = 80
strURL = "http://server/application/images/image1.gif"
objHTTP.DLoad (strURL, "c:\test.gif")
response.write objHTTP.Status
Set objHTTP = Nothing
The string (string containing the name of the file to be downloaded)
parsing code is found in the ASP file provided with this article. VB
code is also provided. ASP code contains two ASP pages. Form1.asp is
the initial form and the results from form1.asp are passed to
form2.asp. In form2.asp, an object is created and the values from
form1.asp are passed to the object.
The VB code contains a form, and values that are to be passed to the
object are hard-coded in the form. You can change the values to test
the functionality.
About the Author
S.S. Ahmed is a senior software engineer in a software development
company that specializes in Web application development. To contact
Ahmed with questions or comments, please email him at
ss_ahmed1@hotmail.com.
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