The information in this article applies to:
- Microsoft Visual Basic Control Creation, Learning, Professional, and
Enterprise Editions for Windows, version 5.0
- Microsoft Visual Basic Standard, Professional, and Enterprise Editions,
32-bit only, for Windows, version 4.0
- Microsoft Access versions 7.0, 97
- Microsoft Excel 97 for Windows
- Microsoft PowerPoint 97 for Windows
- Microsoft Word 97 for Windows
- Microsoft Visual C++, 32-bit Editions, version 4.0
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
- 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.
- 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.
- 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.
- 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 Size
Byte 1 byte 1 byte
String * n 1 byte 2 bytes per UNICODE character
Integer 2 bytes 2 bytes
Boolean 2 bytes 2 bytes
String 4 bytes 4 byte pointer - UNICODE data not in structure
Long 4 bytes 4 bytes
Single 4 bytes 4 bytes
Double 4 bytes 8 bytes
Currency 4 bytes 8 bytes
Date 4 bytes 8 bytes
Variant 4 bytes 16 bytes - may point to data not in structure
Object 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
- Create a new Project Workspace of Dynamic-Link Library type and save as
TestType.
- Create a Text file and save as TestType.DEF. Add the file to the
project:
LIBRARY TestType
EXPORTS
FillUDTSafeArray
FillUDTVariable
- 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);
}
- Build the DLL.
- 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
- 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
Additional reference material for extending these examples to include
Variant types can be found at:
http://www.microsoft.com/oledev/olecom/intro.htm
Keywords : GnrlVb kbinterop kbprg
Technology : kbvba
Version : WINDOWS:4.0 5.0 7.0 97
Platform : NT WINDOWS
Issue type : kbhowto
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