windows - 在 Visual Basic 6 中访问动态加载的 DLL(使用 LoadLibrary)

Question

我需要为 DLL 创建一个包装器，根据需要加载和卸载它(对于那些对这个问题的背景感兴趣的人，请参阅 How to work around memory-leaking 3rd party DLL (no source code) accessed by Tomcat application?)。我在 Visual Basic 6 中执行此操作，并且使用以下示例进行加载和卸载:

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 Sub cmdTestLoadingDLL_Click()

    Dim lb As Long, pa As Long    
    lb = LoadLibrary("D:\projects\other\VB_DLLs\TestDLL\TestDLL.dll")    

    Msgbox "Library address: " + lb

    FreeLibrary lb    

End Sub

我可以看到使用 Process Explorer当显示消息框时，DLL 被加载到内存中，然后被丢弃。然而，调用方法自然是不够的——我需要访问动态加载的 DLL 中的方法。

我怎样才能做到这一点？我想在 TestDLL 中的类 mainClass 中调用方法 getVersion，如下所示:

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 Sub cmdTestLoadingDLL_Click()

    Dim lb As Long, pa As Long
    Dim versionString As String

    lb = LoadLibrary("D:\projects\other\VB_DLLs\TestDLL\TestDLL.dll")    

    versionString = "- From DLL: " + mainClass.getVersion
    MsgBox versionString

    FreeLibrary lb    

End Sub

但是，行

versionString = "- From DLL: " + mainClass.getVersion

抛出错误“需要对象”。

Answer 1

首先，由于您是通过 LoadLibrary 调用它，因此此处没有类 - 仅导出函数供公众使用。所以你的 mainClass 引用永远不会工作。假设您有一个导出的函数 getVersion。

我会尝试以下方法:

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 Sub Foo
  On Error Resume Next

  Dim lb As Long, pa As Long
  Dim versionString As String
  Dim retValue as Long

  lb = LoadLibrary("D:\projects\other\VB_DLLs\TestDLL\TestDLL.dll")  

  'retrieve the address of getVersion'
  pa = GetProcAddress(lb, "getVersion")

  'Call the getVersion function'
  retValue = CallWindowProc (pa, Me.hWnd, "I want my version", ByVal 0&, ByVal 0&)

  'release the library'
  FreeLibrary lb
End Sub