How To Pass Binary Data Between an ActiveX Control and VB

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This article will demonstrate how to pass an array of bytes between Visual Basic and an ActiveX (OLE) Control. This is accomplished by creating a method that takes a VARIANT parameter that will contain a SafeArray of bytes.

More information

Exchanging data in this manner is useful for both raw data and as a technique for passing data structures.

WARNING: Passing data structures in this manner can pose problems if it contains pointers.

Sample Code

   /* Compile options needed : None

   // Automation method in the control BinData.
   void CBinDataCtrl::VBtoVCtoVB(const VARIANT FAR& Buffer)
      // Verify the Variant contains SafeArray of Bytes
      if (Buffer.vt == (VT_ARRAY | VT_UI1)) {
         long Dims = SafeArrayGetDim(Buffer.parray);
         long UpperBounds;
         long LowerBounds;

         if (Dims == 1) {
            SafeArrayGetLBound(Buffer.parray, 1, &LowerBounds);
            SafeArrayGetUBound(Buffer.parray, 1, &UpperBounds);

            // Use LowerBounds and UpperBounds to force a specific
            // Array size as shown here or they can be used to
            // dynamically create the buffer.
            if ((LowerBounds == 0) && (UpperBounds == 512)) {
               // Reference pointer for accessing the SafeArray
               unsigned char* buff;
               // Variable to store the data from the SafeArray
               // could be a global variable or member of CBinDataCtrl
               // Shown as a local variable for demonstration purpose only
               unsigned char m_abBinaryData[512];
               SafeArrayAccessData (Buffer.parray, (void**)&buff);
               for (int i = 0; i < 512; i++) {
                  // Handle the binary data in the buffers
                  // Copying the data passed from VB to VC.
                  m_abBinaryData[i] = buff[i];

                  // Modifying the data to be passed back to VB
                  buff[i] = 0;
               SafeArrayUnaccessData (Buffer.parray);
      AfxMessageBox ("Invalid parameter passed in VBtoVC method.\n"  +
                     "The array may only have a single dimension.\n" +
                     "The array must contain 512 bytes.");
NOTE: Code for Visual Basic 4.0 where BinData is the name of the control:
      Private Sub Form_Load()
          Dim buf(512) As Byte

          For i = 0 To 511
              buf(i) = 50
          Next i

          BinData1.VBtoVCtoVB buf
      End Sub


For more information, please see the following articles in the Microsoft Knowledge Base:
122287 Limits of VB 3.0 & Disptest as Automation Controllers

131046 SAMPLE: BINARY: Transfer Binary Data Using OLE Automation

131086 SAMPLE: SAFEARAY: Use of Safe Arrays in Automation

140202 SAMPLE: MFCARRAY: Using Safe Arrays in MFC Automation

122289 Passing Structures in OLE Automation
Microsoft Systems Journal, June 1996, "Q&A OLE" by Don Box.


Article ID: 154172 - Last Review: October 26, 2013 - Revision: 3.0
Applies to
  • Microsoft Visual C++ 2.0 Professional Edition
  • Microsoft Visual C++ 4.2 Enterprise Edition
kbnosurvey kbarchive kbctrl KB154172

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