How To Pass Binary Data Between an ActiveX Control and VB

The information in this article applies to:

• Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 4.1, 4.2

SUMMARY

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);
               return;
            }
         }
      }
      AfxMessageBox ("Invalid parameter passed in VBtoVC method.\n"  +
                     "The array may only have a single dimension.\n" +
                     "The array must contain 512 bytes.");
   }

      Private Sub Form_Load()
          Dim buf(512) As Byte

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

          BinData1.VBtoVCtoVB buf
      End Sub

REFERENCES

For more information, please see the following articles in the Microsoft Knowledge Base:

   ARTICLE-ID: Q122287
   TITLE     : Limits of VB 3.0 & Disptest as Automation Controllers

   ARTICLE-ID: Q131046
   TITLE     : SAMPLE: BINARY: Transfer Binary Data Using OLE Automation

   ARTICLE-ID: Q131086
   TITLE     : SAMPLE: SAFEARAY: Use of Safe Arrays in Automation

   ARTICLE-ID: Q140202
   TITLE     : SAMPLE: MFCARRAY: Using Safe Arrays in MFC Automation

   ARTICLE-ID: Q122289
   TITLE     : Passing Structures in OLE Automation