Retrieving Data Using Visual Basic (Windows)

Retrieving Data Using Visual Basic

This topic demonstrates how to write a Microsoft Visual Basic application that obtains the binary data from a Web site using Microsoft Windows HTTP Services (WinHTTP).

Prerequisites and Requirements
Retrieving Data Synchronously
- Analyzing the Code

Prerequisites and Requirements

In addition to a working knowledge of Visual Basic, this example requires the following:

The current version of the Platform Software Development Kit (SDK).
The proxy configuration tool to establish the proxy settings for Microsoft Windows HTTP Services (WinHTTP), if your connection to the Internet is through a proxy server. See ProxyCfg.exe, a Proxy Configuration Tool for more information.
A familiarity with network terminology and concepts.

Retrieving Data Synchronously

This section describes how to create a Visual Basic application that synchronously retrieves data over a network. It retrieves a binary image file and displays it in a user interface.

Begin by opening Visual Basic. Open a new Standard EXE project. The new project contains a form named "Form1".

The application uses some of the methods and properties of the WinHttpRequest object. Use early binding to access this object by completing the following two steps.

From the Project menu, click References.
On the list of Available References, select Microsoft WinHttpRequest Component, version 5.0 and click OK.

If the WinHttpRequest object is not available on the Available References list, WinHttp.dll has not been properly installed or is not registered. When you use early binding with a Component Object Model (COM) object, Visual Basic is aware of the methods and properties for that object and can provide assistance with syntax checking. It is also able to call the methods and properties faster.

Create a user interface for the application by using the tools provided in Visual Basic. The form should include a CommandButton named "Command1", a TextBox named "Text1", and a PictureBox named "Picture1". Arrange the controls on the form so that the form is similar to the following image.

Visual Basic Form

Copy the following source code in the code area of the project:


Dim WinHttpReq As WinHttp.WinHttpRequest

Private Sub Form_Load()
    ' Create an instance of the WinHTTPRequest ActiveX object.
    Set WinHttpReq = New WinHttpRequest
End Sub

Private Sub Command1_Click()
    ' Create an array to hold the response data.
    Dim d() As Byte
    
    ' Assemble an HTTP Request.
    WinHttpReq.Open "GET", _
      "http://www.microsoft.com/library/homepage/images/ms-banner.gif",_ 
      False

    ' Send the HTTP Request.
    WinHttpReq.Send
    
    ' Put status and content type into status text box.
    Text1.Text = WinHttpReq.Status & " - " & WinHttpReq.StatusText
  
    ' Put response data into a file.
    Open "temp.gif" For Binary As #1
    d() = WinHttpReq.ResponseBody
    Put #1, 1, d()
    Close
    
    ' Load the data file as a picture.
    Picture1.Picture = LoadPicture("temp.gif")
End Sub

You now have a program that uses a WinHttpRequest object to obtain the Ms-banner.gif file from the Web page at www.microsoft.com. When you run the application and click the CommandButton, the status code is displayed in the TextBox and the resource is displayed as an image in the PictureBox.

Analyzing the Code

The first line of the sample source code is a "Dim" statement, which creates a variable to store an instance of the WinHttpRequest object. This is done at the module level so that the object is available to all the routines in this form module. The instance of the WinHttpRequest object is created in the Load event of the form:


    'Create an instance of the WinHTTPRequest ActiveX object
    Set WinHttpReq = New WinHttpRequest

When Visual Basic encounters this line, it creates an instance of this object. If you get the error value, "ActiveX component can't create object," the WinHttp.dll was not properly registered or is not present on the system.

The handler for the Click event of the CommandButton contains the rest of the source code. The "Dim"statement defines a data array to temporarily hold the data from ResponseBody. Next, the Open method of the WinHttpRequest object assembles an HTTP request:


    ' Assemble an HTTP Request.
    WinHttpReq.Open "GET", _
      "http://www.microsoft.com/library/homepage/images/ms-banner.gif", _
      False

Three parameters specify which HTTP verb to use, the name of the resource, and whether to use WinHTTP synchronously or asynchronously. In this example, the method is using the HTTP verb "GET" to obtain data from http://www.microsoft.com. Specifying FALSE for the last parameter determines that the transaction occurs synchronously. The Open method does not establish a connection to the resource as the name might imply. Rather, it initializes the internal data structures that maintain information about the session, connection, and request.

The Send method assembles the request headers and sends the request. When called in synchronous mode, the Send method also waits for a response before allowing the application to continue:


    ' Send the HTTP request.
    WinHttpReq.Send

Execution of the script pauses while the entire body of the resource is retrieved. After the Send method, the program accesses the status code and status text and displays them in the TextBox:


    'Put status and content type into status text box.
    Text1.Text = WinHttpReq.Status & " - " & WinHttpReq.StatusText

Since binary data cannot be loaded directly into a PictureBox control, the program opens a temporary file to hold the data:


    ' Put response data into a file.
    Open "temp.gif" For Binary As #1

The program then obtains the response by accessing the ResponseBody property of the WinHttpRequest object. Since ResponseBody returns an array of unsigned bytes, the data must be received by a byte array:


    d() = WinHttpReq.ResponseBody

The application places the data returned by the ResponseBody property into the data array, "d()", where it is written to the temporary file using the Put statement. The file is then closed. The picture is then loaded from the temporary file into the PictureBox control, where it is displayed to the user:


    'Load temp.gif to Picture1.
    Picture1.Picture = LoadPicture("temp.gif")

The WinHttpRequest object ensures that any internal resources that are allocated for the HTTP transaction are released.

Prerequisites and Requirements

Retrieving Data Synchronously

Analyzing the Code

See Also