BUG: Visual Basic App Crashes Passing UDT with String and Enum

• Microsoft Visual Basic Professional and Enterprise Editions for Windows, version 6.0
• Microsoft Visual Basic Standard, Professional, and Enterprise Editions, 32-bit only, for Windows, versions 4.0, 5.0

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

Keywords          : kbDLL kbVBp400bug kbVBp500bug kbVBp600bug kbVC kbGrpVB 
Version           : WINDOWS:4.0,5.0,6.0
Platform          : WINDOWS 
Issue type        : kbbug 

Last Reviewed: June 2, 1999