Microsoft KB Archive/175474

From BetaArchive Wiki
Knowledge Base

INFO: Header Required Posting HTML Form Encoded Data to ASP Page

Article ID: 175474

Article Last Modified on 8/18/2005


  • Microsoft Internet Explorer 4.0 128-Bit Edition
  • Microsoft Internet Explorer 4.01 Service Pack 2
  • Microsoft ActiveX SDK
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition

This article was previously published under Q175474


When writing an Internet client application that posts HTML form data to an ASP page using Wininet API or Visual Basic Internet Transfer Control, it is necessary to add the following HTTP header in the POST request:

Content-Type: application/x-www-form-urlencoded

This header indicates that the data being posted is encoded according to the rules in the HTML specification (RFC 1866). ASP will not retrieve your HTML form variables if this header is not present in the POST request.

The following Visual Basic example demonstrates adding a custom HTTP header to a POST request using either Wininet API or the Internet Transfer Control (SP2). Please note that you need the Internet Transfer Control from at least Visual Basic 5.0 Service Pack 2 or Visual Studio Service Pack 2 to implement POST. The original control cannot post correctly due to a bug. The code fragment in the CommandButton1 Click event in the following example uses Wininet API. The code fragment in the CommandButton2 Click event uses Internet Transfer Control (SP2).

Step-by-Step Example

  1. Start a new Standard EXE project. Form1 is added by default.
  2. From the Project menu, choose Components, and select the Internet Transfer Control(SP2) into the project.
  3. Place an Internet Transfer Control 5.0(SP2), Inet1, on Form1.
  4. Place two CommandButtons, Command1 and Command2, on Form1.
  5. Add the following code to the project code window:

          Private Declare Function InternetOpen Lib "wininet.dll" _
             Alias "InternetOpenA" _
                (ByVal lpszCallerName As String, _
                 ByVal dwAccessType As Long, _
                 ByVal lpszProxyName As String, _
                 ByVal lpszProxyBypass As String, _
                 ByVal dwFlags As Long) As Long
          Private Declare Function InternetConnect Lib "wininet.dll" _
                Alias "InternetConnectA" _
                (ByVal hInternetSession As Long, _
                 ByVal lpszServerName As String, _
                 ByVal nProxyPort As Integer, _
                 ByVal lpszUsername As String, _
                 ByVal lpszPassword As String, _
                 ByVal dwService As Long, _
                 ByVal dwFlags As Long, _
                 ByVal dwContext As Long) As Long
       Private Declare Function InternetReadFile Lib "wininet.dll" _
                (ByVal hFile As Long, _
                 ByVal sBuffer As String, _
                 ByVal lNumBytesToRead As Long, _
                 lNumberOfBytesRead As Long) As Integer
       Private Declare Function HttpOpenRequest Lib "wininet.dll" _
                Alias "HttpOpenRequestA" _
                (ByVal hInternetSession As Long, _
                 ByVal lpszVerb As String, _
                 ByVal lpszObjectName As String, _
                 ByVal lpszVersion As String, _
                 ByVal lpszReferer As String, _
                 ByVal lpszAcceptTypes As Long, _
                 ByVal dwFlags As Long, _
                 ByVal dwContext As Long) As Long
       Private Declare Function HttpSendRequest Lib "wininet.dll" _
                Alias "HttpSendRequestA" _
                (ByVal hHttpRequest As Long, _
                 ByVal sHeaders As String, _
                 ByVal lHeadersLength As Long, _
                 ByVal sOptional As String, _
                 ByVal lOptionalLength As Long) As Boolean
       Private Declare Function InternetCloseHandle Lib "wininet.dll" _
                (ByVal hInternetHandle As Long) As Boolean
       Private Declare Function HttpAddRequestHeaders Lib "wininet.dll" _
                 Alias "HttpAddRequestHeadersA" _
                 (ByVal hHttpRequest As Long, _
                 ByVal sHeaders As String, _
                 ByVal lHeadersLength As Long, _
                 ByVal lModifiers As Long) As Integer
       Private Sub Command1_Click()
                Dim hInternetOpen As Long
                Dim hInternetConnect As Long
                Dim hHttpOpenRequest As Long
                Dim bRet As Boolean
                hInternetOpen = 0
                hInternetConnect = 0
                hHttpOpenRequest = 0
                'Use registry access settings.
                Const INTERNET_OPEN_TYPE_PRECONFIG = 0
                hInternetOpen = InternetOpen("http generic", _
                                INTERNET_OPEN_TYPE_PRECONFIG, _
                                vbNullString, _
                                vbNullString, _
                If hInternetOpen <> 0 Then
                   'Type of service to access.
                   Const INTERNET_SERVICE_HTTP = 3
                   Const INTERNET_DEFAULT_HTTP_PORT = 80
                   'Change the server to your server name
                   hInternetConnect = InternetConnect(hInternetOpen, _
                                      "yourserver", _
                                      INTERNET_DEFAULT_HTTP_PORT, _
                                      vbNullString, _
                                      "HTTP/1.0", _
                                      INTERNET_SERVICE_HTTP, _
                                      0, _
                   If hInternetConnect <> 0 Then
                    'Brings the data across the wire even if it locally cached.
                     Const INTERNET_FLAG_RELOAD = &H80000000
                     hHttpOpenRequest = HttpOpenRequest(hInternetConnect, _
                                         "POST", _
                                         "/scripts/testpost.asp", _
                                         "HTTP/1.0", _
                                         vbNullString, _
                                         0, _
                                         INTERNET_FLAG_RELOAD, _
                      If hHttpOpenRequest <> 0 Then
                         Dim sHeader As String
                         Const HTTP_ADDREQ_FLAG_ADD = &H20000000
                         Const HTTP_ADDREQ_FLAG_REPLACE = &H80000000
               sHeader = "Content-Type: application/x-www-form-urlencoded" _
                           & vbCrLf
                         bRet = HttpAddRequestHeaders(hHttpOpenRequest, _
                           sHeader, Len(sHeader), HTTP_ADDREQ_FLAG_REPLACE _
                           Or HTTP_ADDREQ_FLAG_ADD)
                         Dim lpszPostData As String
                         Dim lPostDataLen As Long
                         lpszPostData = "lname=Doe&fname=John"
                         lPostDataLen = Len(lpszPostData)
                         bRet = HttpSendRequest(hHttpOpenRequest, _
                                vbNullString, _
                                0, _
                                lpszPostData, _
                         Dim bDoLoop             As Boolean
                         Dim sReadBuffer         As String * 2048
                         Dim lNumberOfBytesRead  As Long
                         Dim sBuffer             As String
                         bDoLoop = True
                         While bDoLoop
                          sReadBuffer = vbNullString
                          bDoLoop = InternetReadFile(hHttpOpenRequest, _
                             sReadBuffer, Len(sReadBuffer), lNumberOfBytesRead)
                          sBuffer = sBuffer & _
                               Left(sReadBuffer, lNumberOfBytesRead)
                          If Not CBool(lNumberOfBytesRead) Then bDoLoop = False
                         MsgBox sBuffer
                         bRet = InternetCloseHandle(hHttpOpenRequest)
                      End If
                      bRet = InternetCloseHandle(hInternetConnect)
                   End If
                   bRet = InternetCloseHandle(hInternetOpen)
                End If
       End Sub
       Private Sub Command2_Click()
             Dim strURL As String, strFormData As String
             'Change the server to your server name
             strURL = "http://yourserver/scripts/testpost.asp"
             strFormData = "lname=Doe&fname=John"
             Inet1.Execute strURL, "Post", strFormData, _
                "Content-Type: application/x-www-form-urlencoded"
       End Sub
       Private Sub Inet1_StateChanged(ByVal State As Integer)
           Dim vtData As Variant ' Data variable.
           Select Case State
           ' ... Other cases not shown.
           Case icError ' 11
               ' In case of error, return ResponseCode and
       ' ResponseInfo.
               vtData = Inet1.ResponseCode & ":" & _
           Case icResponseCompleted  ' 12
               Dim strData As String: strData = ""
               Dim bDone As Boolean: bDone = False
               ' Get first chunk.
               vtData = Inet1.GetChunk(1024, icString)
               Do While Not bDone
                   strData = strData & vtData
                   ' Get next chunk.
                   vtData = Inet1.GetChunk(1024, icString)
                   If Len(vtData) = 0 Then
                       bDone = True
                   End If
              MsgBox strData
           End Select
       End Sub
       Private Sub Form_Load()
       End Sub
  6. From the File menu, choose Make Project1.exe to compile the project.

    Note that tracing the Inet1_StateChanged event callback function in the VB debugger may not work properly because this callback function needs to be repeated called and can't be stopped in debugger before a complete HTTP response is received. If you need to trace the Inet1_StateChanged event callback function in the debugger, use Debug.Print.
  7. Create a testpost.asp file and place it in your Web server scripts directory:

          <TITLE>test asp</TITLE>
          <H3>test asp</H3>
          fname is
          <% =Request("fname") %>
          lname is
          <% =Request("lname") %>


RFC 1866 - Hypertext Markup Language - 2.0 at:

Section 8.2, "Form Submission"

Keywords: kbinfo kbcode KB175474