HOWTO: Pass a SafeArray of Strings in a VARIANT*

ID: Q167668

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0, 4.0a, 4.1, 4.2, 4.2b, 5.0, 6.0

SUMMARY

When you look at the information available on passing SafeArrays between Visual C++ and Visual Basic, most pass the SafeArray in a parameter of type VARIANT. In MFC this translates into a "const VARIANT& varName" parameter.

As an alternative, this article explores how to pass a VARIANT*, which in MFC will be declared as a "VARIANT FAR* varName" parameter.

Why should you use one method over the other? Because most applications allow modifications to a SafeArray passed in a VARIANT, but not all. In Excel 95, you can pass a SafeArray in a VARIANT to a Visual C++ server, modify the values in the server, return the modified SafeArray, and there is no change in Excel 95. Another reason to use this technique is to work around a limitation in Visual Basic when you use early binding.

MORE INFORMATION

To demonstrate this concept, start with a new "MFC AppWizard(exe)" project called StrArray. In "Step 3 of 6," select Automation, and then click Finish and OK to generate the project files.

Now create a single automation method using ClassWizard. In ClassWizard, select "CStrArrayDoc" under "Class name:". Click the Automation tab and click Add Method. Enter the following values into the Add Method dialog box:


   External Name : Sort
   Internal Name : Sort
   Return Type   : long
   Parameter List:

      Name       : vArray
      Type       : VARIANT*

Click OK and Edit Code.

To implement the Sort method, use the FastSort method of the MFCARRAY sample mentioned in the REFERENCES section below. This allows you to see the differences between passing the parameter as a "const VARIANT& varName" and a "VARIANT* varName". Modify the Sort method as follows:


   ...
   #include <afxpriv.h>
   ...
   long CStrArrayDoc::Sort(VARIANT FAR* vArray)
   {

      long i, j, min;
      BSTR bstrTemp;
      SAFEARRAY FAR* psa = NULL;
      BSTR HUGEP *pbstr;
      HRESULT hr;
      DWORD dwTimeStart;
      LONG cElements, lLBound, lUBound;

      USES_CONVERSION;

      // Type check VARIANT parameter. It should contain a BSTR array
      // passed by reference. The array must be passed by reference it is
      // an in-out-parameter.
      if (V_VT(vArray) != (VT_ARRAY | VT_BSTR))
         AfxThrowOleDispatchException(1001,
           "Type Mismatch in Parameter. Pass a string array by reference");
      psa = V_ARRAY(vArray);
      // Check dimensions of the array.
      if (SafeArrayGetDim(psa) != 1)
         AfxThrowOleDispatchException(1002,
           "Type Mismatch in Parameter. Pass a one-dimensional array");

      dwTimeStart = GetTickCount();

      // Get array bounds.
      hr = SafeArrayGetLBound(psa, 1, &lLBound);
      if (FAILED(hr))
          goto error;
      hr = SafeArrayGetUBound(psa, 1, &lUBound);
      if (FAILED(hr))
          goto error;

      // Get a pointer to the elements of the array.
      hr = SafeArrayAccessData(psa, (void HUGEP* FAR*)&pbstr);
      if (FAILED(hr))
         goto error;

      // Bubble sort.
      cElements = lUBound-lLBound+1;
      for (i = 0; i < cElements-1; i++)
      {
         min = i;
         for (j = i+1; j < cElements; j++)
         {
            // NULL is a valid value for a BSTR. This code treats a NULL
            // BSTR as less than other string values.
            if (pbstr[min] == NULL)
               continue;
            else if (pbstr[j] == NULL
               || wcscmp(pbstr[j], pbstr[min]) < 0)
               min = j;
         }

         //Swap array[min] and array[i].
         bstrTemp = pbstr[min];
         pbstr[min] = pbstr[i];
         pbstr[i] = bstrTemp;
      }

      hr = SafeArrayUnaccessData(psa);
      if (FAILED(hr))
         goto error;

      return GetTickCount()-dwTimeStart;

   error:

      AfxThrowOleDispatchException(1003,
        "Unexpected Failure in FastSort method");
      return 0;

   }

Now you can build and test the automation server. After you build the server, run it as a stand-alone to register it. Run Visual Basic and create a new Visual Basic project. Place a button on the form and modify the handler to match the code below:


   Private Sub Command1_Click()
      Dim o As Object
      Dim v As Variant
      ReDim v(50) As String
      Dim SortTime As Long

      Set o = CreateObject("StrArray.Document")

      upperbound = 1
      lowerbound = 100
      For n = 0 To 50
          v(n) = "Entry " & Int((upperbound-lowerbound+1)*Rnd+lowerbound)
      Next n

      SortTime = o.Sort(v)
      MsgBox ("Done")
   End Sub

The key to getting this syntax to work in Visual Basic is to first Dim the array variable as a variant, and then ReDim it so that it becomes a variant containing an array of strings. Now you are ready to test the server. Set break points in the Visual Basic code before and after the call to Sort. Run the Visual Basic application, click Command1 and use the watch window to check the values for v.

REFERENCES

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

Q122287 Limits of VB 3.0 Disptest as Automation Controllers

Q131046 SAMPLE: BINARY: Transfer Binary Data Using OLE Automation

Q131086 SAMPLE: SAFEARAY: Use of Safe Arrays in Automation

Q140202 SAMPLE: MFCARRAY: Using Safe Arrays in MFC Automation

Q122289 Passing Structures in OLE Automation

Q154172 How to Pass Binary Data Between an ActiveX Control and VB

Additional query words:

Keywords : kbole kbAutomation kbMFC kbVC200 kbVC210 kbVC220 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbfaq
Version : winnt:2.0,2.1,2.2,4.0,4.0a,4.1,4.2,4.2b,5.0,6.0
Platform : winnt
Issue type : kbhowto