interop - File not found when loading dll from vb6

Question

I am declaring and calling a dll function using the following syntax in VB6:

'Declare the function
Private Declare Sub MYFUNC Lib "mylib.dll" ()

'Call the function
MYFUNC

Calling the function results in the error File not found: mylib.dll. This happens when the application is run from the vb6 IDE or from a compiled executable.

The dll is in the working directory, and I have checked that it is found using ProcMon.exe from sysinternals. There are no failed loads, but the Intel Fortran dlls are not loaded (the ProcMon trace seems to stop before then).

I have also tried running the application in WinDbg.exe, and weirdly, it works! There are no failures on this line. The ProcMon trace shows that the Intel Fortran dlls are loaded when the program is run in this way.

The dll is compiled with Fortran Composer XE 2011.

Can anyone offer any help?

Answer 1

When loading DLLs, "file not found" can often be misleading. It may mean that the DLL or a file it depends on is missing - but if that was the case you would have spotted the problem with Process Monitor.

Often, the "file not found" message actually means that the DLL was found, but an error occured when loading it or calling the method.

There are actually three steps to calling a procedure in a DLL:

1. Locate and load the DLL, running the DllMain method if present.
2. Locate the procedure in the DLL.
3. Call the procedure.

Errors can happen at any of these stages. VB6 does all this behind the scenes so you can't tell where the error is happening. However, you can take control of the process using Windows API functions. This should tell you where the error is happening. You can alse set breakpoints and use Process Monitor to examine your program's behaviour at each point which may give you more insights.

The code below shows how you can call a DLL procedure using the Windows API. To run it, put the code into a new module, and set the startup object for your project to "Sub Main".

Option Explicit

' Windows API method declarations
Private Declare Function FreeLibrary Lib "kernel32" (ByVal hLibModule As Long) As Long
Private Declare Function LoadLibrary Lib "kernel32" Alias "LoadLibraryA" (ByVal lpLibFileName As String) As Long
Private Declare Function GetProcAddress Lib "kernel32" (ByVal hModule As Long, ByVal lpProcName As String) As Long
Private Declare Function CallWindowProc Lib "user32" Alias _
    "CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal hWnd As Long, _
    ByVal Msg As Any, ByVal wParam As Any, ByVal lParam As Any) _
    As Long

Private Declare Function FormatMessage Lib "kernel32" Alias _
    "FormatMessageA" (ByVal dwFlags As Long, lpSource As Long, _
    ByVal dwMessageId As Long, ByVal dwLanguageId As Long, _
    ByVal lpBuffer As String, ByVal nSize As Long, Arguments As Any) _
    As Long

Const FORMAT_MESSAGE_FROM_SYSTEM = &H1000

Const MyFunc As String = "MYFUNC"
Const MyDll As String = "mylib.dll"

Sub Main()

    ' Locate and load the DLL. This will run the DllMain method, if present
    Dim dllHandle As Long
    dllHandle = LoadLibrary(MyDll)

    If dllHandle = 0 Then
        MsgBox "Error loading DLL" & vbCrLf & ErrorText(Err.LastDllError)
        Exit Sub
    End If

    ' Find the procedure you want to call
    Dim procAddress As Long
    procAddress = GetProcAddress(dllHandle, MyFunc)

    If procAddress = 0 Then
        MsgBox "Error getting procedure address" & vbCrLf & ErrorText(Err.LastDllError)
        Exit Sub
    End If

    ' Finally, call the procedure
    CallWindowProc procAddress, 0&, "Dummy message", ByVal 0&, ByVal 0&

End Sub

' Gets the error message for a Windows error code
Private Function ErrorText(errorCode As Long) As String

    Dim errorMessage As String
    Dim result As Long

    errorMessage = Space$(256)
    result = FormatMessage(FORMAT_MESSAGE_FROM_SYSTEM, 0&, errorCode, 0&, errorMessage, Len(errorMessage), 0&)

    If result > 0 Then
        ErrorText = Left$(errorMessage, result)
    Else
        ErrorText = "Unknown error"
    End If

End Function