Q221101: BUG: Visual Basic App Crashes Passing UDT w/ String & Enum
Article: Q221101
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0,5.0,6.0
Operating System(s):
Keyword(s): kbDLL kbVBp400bug kbVBp500bug kbVBp600bug kbVC kbGrpDSVB
Last Modified: 11-JAN-2001
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The information in this article applies to:
- Microsoft Visual Basic Professional Edition for Windows, version 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0
- Microsoft Visual Basic Standard Edition, 32-bit, for Windows, versions 4.0, 5.0
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, versions 4.0, 5.0
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, versions 4.0, 5.0
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SYMPTOMS
========
If a Visual Basic application retrieves a User Defined Type (UDT) containing
String and Enum fields from a C++ DLL, the Visual Basic application crashes on
the call to the DLL file. No error message is displayed.
RESOLUTION
==========
Use the Long data type instead of Enum in the Type declaration of the Visual
Basic UDT.
STATUS
======
Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.
MORE INFORMATION
================
Steps to Reproduce Behavior
---------------------------
1. Start Visual C++ 6.0. From the File menu, choose New.
2. Select the Projects tab. Select Win32 Dynamic-Link Library from the list, and
then enter MyDll in the Project Name field. Click OK.
3. In the Win32 Dynamic-Link Library dialog box, select the An empty DLL project
option, and click Finish. Click OK in the following dialog box.
4. From the File menu, choose New. Select the Files tab, choose C++ Source File
from the list, and then enter MyDll.cpp in the File Name field. Click OK.
5. Paste the following code in the MyDll.cpp pane:
#include <windows.h>
// the UDT declaration
enum VBenum
{
wf1 = 1,
wf8 = 8,
wf1e = 0x1E
};
typedef struct
{
char cArray[30];
VBenum lformat;
} mUDT;
// dll's export method
int _stdcall passUDT(mUDT* pU)
{
char* cp="hello, world!";
strncpy(pU->cArray, cp, 14);
pU->lformat = wf1e;
return 30;
}
6. From the File menu, click New. Select the Files tab, and then choose Text
File from the list. Enter MyDll.def in the File Name field.
Paste the following code in the MyDll.def pane:
; The DEF File
LIBRARY MyDll
EXPORTS
passUDT @1
7. From the Build menu, choose Rebuild All. Exit Visual C++.
8. Start a new Visual Basic project. Form1 is created by default.
9. Add two CommandButtons to Form1.
10. Copy the following code into the General Declarations section of Form1:
Private Enum MyEnum
wf1 = &H1&
wf8 = &H8&
wf1e = &H1E&
End Enum
Private Type MyUDT1
myStr As String * 30
mL As MyEnum
End Type
Private Type MyUDT2
myStr As String * 30
mL As Long
End Type
Private Declare Function passUDT1 Lib "c:\MyDll\Debug\MyDll.dll" _
Alias "passUDT" (myU As MyUDT1) As Long
Private Declare Function passUDT2 Lib "c:\MyDll\Debug\MyDll.dll" _
Alias "passUDT" (myU As MyUDT2) As Long
' Visual Basic crashes on the call to passUDT1.
Private Sub Command1_Click()
Dim mU As MyUDT1
MsgBox passUDT1(mU)
MsgBox mU.myStr
End Sub
' Visual Basic works correctly on the call to passUDT2.
Private Sub Command2_Click()
Dim mU As MyUDT2
MsgBox passUDT2(mU)
MsgBox mU.myStr
End Sub
11. Modify the two Declare statements in the preceding code example so that they
point to the location of the MyDll.dll file on your system.
12. Run the project and click Command2. Two Message Boxes display to confirm
that no error occurred.
13. Click Command1 and Visual Basic crashes without an error message.
Additional query words: Structure
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Keywords : kbDLL kbVBp400bug kbVBp500bug kbVBp600bug kbVC kbGrpDSVB
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600 kbVB400Search kbVB400
Version : WINDOWS:4.0,5.0,6.0
Issue type : kbbug
Solution Type : kbpending
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