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How To Fill a 32-bit VBA Array of UDType via a Visual C++ DLL

This article was previously published under Q171583
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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.
SUMMARY
This article provides sample DLL code in Microsoft Visual C++ 4.x that illustrates how to fill a 32-bit VBA array of a user-defined type (UDT) containing fixed-length strings with data. It also provides VBA code that defines the equivalent structure and how to pass the array to the DLL.
MORE INFORMATION
The Vb4dll.txt file that ships with Visual Basic 4.0 describes both passing a variable of UDT to a DLL and passing an array of String (SAFEARRAY of BSTR) to a DLL, but does not discuss passing an array of UDTs. This article provides an example of a Visual Basic program passing an array of UDTs to a C++ DLL and filling the array with data in the DLL.

NOTE: This article does not address:
  • Resizing the array.
  • UDTs that contain variable length strings, Objects, or Variants.
  • Multi-dimension arrays.

Issues

  1. When passing an array of UDTs with string elements, the strings are not converted from UNICODE to ANSI as when passing a variable of UDT.
  2. Elements of a UDT in Visual Basic use a 4-byte alignment. The default Visual C++ alignment is on 8-byte boundaries, so you have to explicitly set a smaller alignment.
  3. In Visual Basic, UNICODE strings are implemented as an array of bytes; in Visual C++, they are implemented as an array of unsigned short int. This means that Visual Basic aligns UNICODE characters on 1-byte boundaries and Visual C++ aligns them on 2-byte boundaries. Because of this, your Visual C++ code has to specify 1-byte alignment as opposed to 4-byte alignment if you have any fixed-length strings in your UDT. With the 1-byte alignment, you will have to add filler fields in your Visual C++ struct to fix the alignment of other types on a 4-byte boundary.
  4. Visual C++ mangles function names. You will need to build a .DEF file to make the names readable by Visual Basic.
Table of Visual Basic Structure Alignment in SAFEARRAYS:
Type         Alignment   SizeByte          1 byte     1 byteString * n    1 byte     2 bytes per UNICODE characterInteger       2 bytes    2 bytesBoolean       2 bytes    2 bytesString        4 bytes    4 byte pointer - UNICODE data not in structureLong          4 bytes    4 bytesSingle        4 bytes    4 bytesDouble        4 bytes    8 bytesCurrency      4 bytes    8 bytesDate          4 bytes    8 bytesVariant       4 bytes    16 bytes - may point to data not in structureObject        4 bytes    4 byte pointer - object not in structure				
Visual Basic passes arrays as the OLE SAFEARRAY type. This type contains a header structure that contains the following information:
   unsigned short cDims;   // Count of dimensions   unsigned short fFeatures;  // Flags   unsigned long cbElements;  // Size of an element of the array   unsigned long cLocks;   // Lock count   void HUGEP* pvData;     // Pointer to the data   SAFEARRAYBOUND rgsabound[n];  // One bound for each dimension				
The SAFEARRAYBOUND type contains the following elements:
   unsigned long cElements;   // Number of elements in this dimension   long lLbound;     // Lower bound				
The Visual C++ code can read the SAFEARRAY structure and get a pointer to the data (pvData), which contains contiguous data items, and read the SAFEARRAYBOUND structure to determine the number of elements passed.

WARNING: ANY USE BY YOU OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

Step-by-Step Example

  1. Create a new Project Workspace of Dynamic-Link Library type and save as TestType.
  2. Create a Text file and save as TestType.DEF. Add the file to the project:
    LIBRARY   TestType   EXPORTS     FillUDTSafeArray     FillUDTVariable					
  3. Create a Text file and save as TestType.CPP. Add the file to the project:
          #include <windows.h>      #include <ole2.h>      #pragma pack(1)      typedef struct      {      short f1;      short filler_1;      long f2;      BYTE f3;      WCHAR f4;      BYTE filler_2;      float f5;      } MyStruct;      void _stdcall FillUDTVariable(MyStruct * ms)      {      ms->f1 = 2001;      ms->f2 = 20012001;      ms->f3 = 255;      ms->f4 = L'A';      ms->f5 = 200.1f;      }      void _stdcall FillUDTSafeArray(LPSAFEARRAY FAR * ppsa)      {      MyStruct * pdata;      unsigned int i;      pdata = (MyStruct*)((*ppsa)->pvData);      for (i=0;i<((*ppsa)->rgsabound->cElements);i++,pdata++)         FillUDTVariable(pdata);      }					
  4. Build the DLL.
  5. In your VBA application, add a Module with the following code: (You will need to explicitly specify the path of the DLL or move the DLL to the system directory, application directory, or elsewhere on the path.)
       Option Explicit      Type My_VarUDT        F1 As Integer        F2 As Long        F3 As Byte        F4(0 To 1) As Byte  ' to avoid UNICODE/ANSI conversion        F5 As Single      End Type      Type My_ArrayUDT        F1 As Integer        F2 As Long        F3 As Byte        F4 As String * 1        F5 As Single      End Type      Declare Sub FillUDTVariable Lib "TestType.DLL" (A As My_VarUDT)      Declare Sub FillUDTSafeArray Lib "TestType.DLL" (A() As My_ArrayUDT)      Sub Test()      Dim A As Long, B As My_VarUDT, C As String, D(3) As My_ArrayUDT        Debug.Print "---Variable of My_VarUDT-------"        FillUDTVariable B        With B          C = .F4          Debug.Print .F1, .F2, .F3, C; "("; .F4(0); .F4(1); ")", .F5        End With        Debug.Print "---Safe array of My_ArrayUDT-------"        FillUDTSafeArray D()        For A = 0 To 3          With D(A)            Debug.Print .F1, .F2, .F3, .F4; "("; AscB(MidB(.F4, 1, 1));            Debug.Print AscB(MidB(.F4, 2, 1)); ")", .F5          End With        Next A      End Sub					
  6. Save your VBA project and test the code by typing Test in the Immediate Window. The output should appear as follows:
       ---Variable of My_VarUDT-------    2001          20012001      255          A( 65  0 )     200.1   ---Safe array of My_ArrayUDT-------    2001          20012001      255          A( 65  0 )     200.1    2001          20012001      255          A( 65  0 )     200.1    2001          20012001      255          A( 65  0 )     200.1    2001          20012001      255          A( 65  0 )     200.1					
    NOTE: The My_VarUDT example is included to illustrate avoiding the normal UNICODE/ANSI conversion that happens when calling a DLL and passing a single variable. This conversion does not happen when passing the entire array, so can use STRING * 1 in the My_ArrayUDT as opposed to passing a byte array.
REFERENCES
Microsoft Visual C++ Help on SAFEARRAY and related reading

Microsoft Visual Basic file: VB4DLL.TXT

Properties

Article ID: 171583 - Last Review: 01/20/2007 02:18:34 - Revision: 5.2

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