DOCUMENT:Q194609 24-JUL-2001 [vbwin]
TITLE :HOWTO: Pass Array of UDTs with Variable Length Strings to C/C++
PRODUCT :Microsoft Visual Basic for Windows
PROD/VER:WINDOWS:5.0,6.0; winnt:5.0,6.0
OPER/SYS:
KEYWORDS:kbcode kbDLL kbString kbVBp500 kbVBp600 kbVC500 kbVC600 kbGrpDSVB kbCodeSam
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The information in this article applies to:
- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SUMMARY
=======
When you are working with an array of User-Defined types (UDTs) in Visual Basic,
you may want to pass the array to a C/C++ DLL. However, the array cannot be
passed directly because SAFEARRARYs are limited to Automation- safe data types,
and a UDT is not a standard Automation data type.
Moreover, if the UDT contains variable length strings, you cannot pass a pointer
to the first member of the array because Visual Basic will create a temporary
copy of the member in order to handle normal UNICODE to ANSI conversion of
strings.
This article demonstrates one way to bypass these problems and successfully pass
an array of UDTs with variable length strings to C/C++ DLLs.
MORE INFORMATION
================
The solution provided here relies on the type library to prevent Visual Basic
from copying the content of the array to the temporary space and transforming
the strings between ANSI and UNICODE. See the REFERENCES section below for more
information on this topic.
The UDT in this sample is:
Type TestUDT
l As Long
str As String
End Type
Step-by-Step Example - Create the DLL and Type Library
------------------------------------------------------
1. Open Visual C++ and select File|New. On the Projects tab, select "Win32
Dynamic-Link Library" and name the project "UDTArray."
2. Select File|New again. On the Files tab, select "C++ Source File," name the
file "UDTArray.c," and click OK.
3. Repeat step 2 and this time choose "Text File" as the file type. Name the
files "UDTArray.def" and "UDTArray.odl," respectively.
4. Next, add the following code to UDTArray.c:
#include <windows.h>
#include <oleauto.h>
struct tagTestUDT
{
long l;
BSTR str;
} TestUDT;
void __stdcall ModifyStruct(struct tagTestUDT *t, long nTotalItem)
{
// modify the last element
long i = nTotalItem - 1; // the array begins from 0
(t+i)->l = 200;
SysFreeString((t+i)->str ); // free the string passed in
(t+i)->str = SysAllocString(OLESTR("Changed."));
}
5. To make the functions exportable, add the following to UDTArray.def:
EXPORTS
ModifyStruct
6. Declare your functions in a type library by adding the following to
UDTArray.odl:
[
uuid(D6229080-2CC3-11d2-9FD8-00C04F8EF4D4),
helpstring("Pass Array of UDTs Helper")
]
library PassUDTLib
{
//definition of the UDT
typedef struct tagTestUDT
{
long l;
BSTR str;
}TestUDT;
[
helpstring("test"),
dllname("UDTArray.dll")
]
module structDLL
{
[
helpstring("Modify the elements of array"),
entry("ModifyStruct")
]
void _stdcall ModifyStruct
(
[in, out] TestUDT* t,
[in] long nTotalItem
);
};
};
7. Go to Project|Settings, click the C/C++ Tab, and then choose "Code
Generation" from the Category dropdown box. Change the "Struct Member
Alignment" to be "4 Bytes" because Visual Basic uses 4 bytes alignment And
Visual C++ uses 8 bytes by default.
8. Compile your DLL by choosing "Rebuild All" from the Build menu. When
complete, copy the new DLL (UDTArray.dll) to your Visual Basic directory for
testing.
Step-by-Step Example - The Visual Basic Client App
--------------------------------------------------
1. Open Visual Basic and create a new Standard EXE Project. Form1 is created by
default.
2. From the Project menu, select References to call up the References dialog
box, and then click Browse. Find the UDTArray.tlb file generated for your
DLL.
3. Add a CommandButton to Form1 and then add the following code to the code
window for Form1:
Private Sub Command1_Click()
' Note: You do not need to redefine TestUDT because it is in
' the type library we added to our project.
Dim t(0 To 1) As TestUDT
Dim i as long
t(0).l = 1
t(0).str = "test1"
t(1).l = 2
t(1).str = "Test2"
i = UBound(t) - LBound(t) + 1
ModifyStruct t(0), i 't(0)'s address will be passed to C
MsgBox "t(1).l = " & t(1).l & vbCrLf & "t(1).str = " & t(1).str
End Sub
4. Run the Visual Basic application and click the CommandButton. The message box
displayed indicates that the last element of the array has been changed.
REFERENCES
==========
For detailed information on how to create a C DLL and use it in Visual Basic,
please see the following articles:
"DLLs for Beginners" in the Microsoft Developer Network (MSDN) Library
"VB5DLL.DOC" located on the VB5 CD-ROM in the "Tools\Docs" directory
For more information on using a type library to declare C/C++ functions in Visual
Basic, see the following article in the Microsoft Knowledge Base:
Q189133 : HOWTO: Make C DLL More Accessible to VB with a Type Library
For additional information, please see the following articles in the Microsoft
Knowledge Base:
Q142840 : Visual Basic Requirements for Exported DLL Functions
Q171583 : HOWTO: Fill a 32-bit VBA Array of UDType via a Visual C++ DLL
Additional query words:
======================================================================
Keywords : kbcode kbDLL kbString kbVBp500 kbVBp600 kbVC500 kbVC600 kbGrpDSVB kbCodeSam
Technology : kbVCsearch kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600 kbVC500 kbVC600 kbVC32bitSearch kbVC500Search
Version : WINDOWS:5.0,6.0; winnt:5.0,6.0
Issue type : kbhowto
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