How to post data using TCppWebBrowser in C++Builder

By: Adam Vieweger

Abstract: This article demonstrates techniques for using the CppWebBrowser component to browse and post data using Navigate and Navigate2 methods. By Adam Vieweger.

I had been tearing my hair out for more than two weeks. I wanted to use the TCppWebBrowser component (distributed with Borland C++Builder starting with BCB 5 Enterprise Edition) to create an app that would browse the Web and allow users to post data to Web pages. I was having a tough time of it because the documentation on TCppWebBrowser is fairly sparse and there aren't many articles out there to guide me.

So I rolled up my sleeves and started working. I hope my discoveries prove useful to you.

Browsing with CppWebBrowser turned out to be fairly simple. There are two methods that provide  browsing capabilities: Navigate and Navigate2:

CppWebBrowser1->Navigate("http://www.inprise.com")

Navigate2 is extension of Navigate, but practically it does not matter for us -- in this article we can use both of them interchangeably.

Should you want to find more information about WebBrowser component properties, methods, and events, please refer to online MSDN library: http://msdn.microsoft.com/library/default.asp?url=/workshop/browser/webbrowser/reference/Objects/WebBrowser.asp

HARDER PROBLEMS

Posting data proved to be a tougher nut to crack. Microsoft's component requires data to be passed in a SAFEARRAY structure. The problem is that all Navigate2 parameters are TVariant type. I needed to convert between SAFEARRAY and  TVariant.

That's where the Borland Community site came in handy. I found a great tutorial on how to use SAFEARRAYs with C++Builder. For further details please refer to SAFEARRAYs made easier.

The following code demonstrates the results of my research. Navigate requires that the TVariant should be of type VB_ARRAY and that it should point to a SAFEARRAY. The SAFEARRAY should be of element type VT_UI1 (an unsigned int), dimension one, and it should have an element count equal to the number of bytes of data to be posted.

Confused? Take a look at the code and everything should be much clearer:

// *Method 1*
  TVariant vtEmpty;
  TVariant vtPostDataArray;
  char *str = "action=LogMeIn&username=MyName&password=MyPass";

  SAFEARRAY FAR *psa = NULL;
  SAFEARRAYBOUND sabound[48];
  sabound[0].cElements = strlen(str);
  sabound[0].lLbound = 0;
  psa = SafeArrayCreate(VT_UI1, 1, sabound);
  for(unsigned int n = 0; n < strlen(str); n++){
    SafeArrayPutElement(psa, (long*)0, (void*)str[n]);
  }
  
  vtEmpty.vt = VT_EMPTY;
  vtPostDataArray.vt = VT_ARRAY;
  vtPostDataArray.SetSAFEARRAY(psa);
// or vtPostDataArray=psa;

  TVariant vAddress = {"http://my.server/test/postresults.asp"};
  CppWebBrowser1->Navigate2(&vAddress, &vtEmpty, &vtEmpty, &vtPostDataArray, &vtEmpty);
  SafeArrayDestroy(psa);

// *Method 2*
  TVariant vtEmpty;
  char *str = "action=LogMeIn&username=MyName&password=MyPass";
  TSafeArrayDim1 dim(strlen(str));
  TSafeArrayUInt1 uint_array(dim);
  for(unsigned int n = 0; n < strlen(str); n++){
    uint_array[n]=str[n];
  }
 
  SAFEARRAY* sa = uint_array.Detach();
  SafeArrayCopy(sa, &uint_array);

  vtEmpty.vt = VT_EMPTY;
  TVariant vAddress = {"http://my.server/test/postresults.asp"};
  CppWebBrowser1->Navigate2(&vAddress, &vtEmpty, &vtEmpty, &sa, &vtEmpty);
  SafeArrayDestroy(sa);

Method 1 and Method 2 are pretty good code, but they suffer from the same flaw -- they don't work! Both pass the data correctly, but Navigate still did not want to post the data to the specified URL.

DIGGING DEEPER

I headed back to MSDN for more research on WebBrowser capabilities. And I found two articles that helped: Q167658 and Q165800. With that information in hand, I was finally successful!

Here's what I wound up with. This code demonstrates how to use a CppWebBrowser component for browsing and for posting HTTP data. You can easily modify this code for use in your own applications.

void WebPostData(TCppWebBrowser *CppWebBrowser, String sURL, String sPostData)
{
  BSTR bstrHeaders = NULL;
  TVariant vFlags = {0}, vTargetFrameName={0}, vPostData={0}, vHeaders={0};
  LPSAFEARRAY psa;
  LPCTSTR cszPostData = sPostData.c_str();
  UINT cElems = lstrlen(cszPostData);
  LPSTR pPostData;
  LPVARIANT pvPostData;

  bstrHeaders = SysAllocString(L"Content-Type: application/x-www-form-urlencodedrn");
  if (!bstrHeaders){
    Application->MessageBox("Could not allocate bstrHeaders", "Warning", MB_OK | MB_ICONWARNING);
    return;
  }

  V_VT(&vHeaders) = VT_BSTR;
  V_BSTR(&vHeaders) = bstrHeaders;

  pvPostData = vPostData;

  if(pvPostData){
    VariantInit(pvPostData);

    psa = SafeArrayCreateVector(VT_UI1, 0, cElems);
    if(!psa){
      return;
    }

    SafeArrayAccessData(psa, (LPVOID*)&pPostData);
    memcpy(pPostData, cszPostData, cElems);
    SafeArrayUnaccessData(psa);

    V_VT(pvPostData) = VT_ARRAY | VT_UI1;
    V_ARRAY(pvPostData) = psa;
  }

   CppWebBrowser->Navigate((TVariant)sURL, &vFlags, &vTargetFrameName, &vPostData, &vHeaders);
}

I have no reason to think this code is perfect. But it does have a unique advantage over my previous efforts: It works!

I hope it helps you master C--WebBrowser with C++Builder.

--Adam Vieweger

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