PRB: Cannot Simultaneously Write to Disk File and Have It Opened in ActiveX DLL

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, versions 5.0, 6.0

SYMPTOMS

When you attempt to write to the file number of a file that was opened in an ActiveX DLL or vice versa, the following run-time error is generated:

Error '52' Bad File name or number

CAUSE

The file number created when a file is opened cannot be shared between separate projects or executable modules.

RESOLUTION

If a file must be accessed by both an EXE and an associated DLL, wrap all of the file access activities in a class module in the ActiveX DLL.

STATUS

This behavior is by design.

MORE INFORMATION

Steps to Reproduce Behavior

1. Create an ActiveX DLL project. Class1 is added by default.



2. From the Project menu, click Project Properties. Change the Project Name to DllFileNumber and click OK.



3. Add the following code to the General Declarations section of Class1:




Option Explicit

Private m_Fn As Integer

Public Sub WriteFile(Fn As Variant)
   Write #Fn, "This is a text from the DLL."
End Sub

Public Sub OpenFile()
   m_Fn = FreeFile
   Open "C:\DllTEST.txt" For Append As #m_Fn
End Sub

Private Sub Class_Initialize()
   m_Fn = 0
End Sub

Public Property Get DLLFileNum() As Variant
   DLLFileNum = m_Fn
End Property

Public Sub CloseFile()
   If m_Fn <> 0 Then
      Close #m_Fn
      m_Fn = 0
   End If
End Sub 

4. Save the ActiveX Dll project.



5. From the File menu, click Add Project and add a Standard EXE project. Form1 is added by default.



6. In the project window, set the Standard EXE project as the Start Up project by right-clicking on the project name and clicking "Set as Start Up" in the pop-up menu.



7. From the Project menu, click References and check the DllFileNumber entry.



8. Add two CommandButtons to Form1: Command1 and Command2 will be added by default. If a size larger than the default is used, the captions will be more readable.



9. Add the following code to the General Declarations section of Form1:




Option Explicit

Private Sub Command1_Click()
   Dim objvar As New DllFileNumber.Class1
   Dim sVar As String

   sVar = "This is text from the main program."
   objvar.OpenFile
   objvar.WriteFile objvar.DLLFileNum
   Write #objvar.DLLFileNum, sVar 'Error 52 occurs here.
   objvar.CloseFile
End Sub

Private Sub Command2_Click()

   Dim FileNumber As Variant
   Dim objvar As New DllFileNumber.Class1

   FileNumber = FreeFile

   Open "C:\TEST.txt" For Append As #FileNumber
   Write #FileNumber, "This is text from the main program."

   objvar.WriteFile FileNumber  'Error 52 occurs here.

   Close #FileNumber
End Sub

Private Sub Form_Load()
   Command1.Caption = "Write to a file opened in the DLL"
   Command2.Caption = "Write to a file opened in the main program"
End Sub 

10. Run the program and click on the button labeled "Write to a file opened in the DLL." A Run-Time error 52 will occur on the line of Write statement. End the program.



11. Run the program again and click on the button labeled "Write to a file opened in the main program." A Run-Time error 52 will occur on the line containing the WriteFile method.

Resolution

1. Comment out the code lines containing the following comment:




' Error 52 occurs here. 

2. Run the program again and click each CommandButton. No error will occur.

Additional query words: kbGrpVB

Keywords : kbsample kbActiveX kbDLL kbVBp kbVBp500 kbVBp600 kbGrpVB
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbprb