VB6聲明C++函數提供了「錯誤的DLL調用約定」

Question

什麼是正確的VB6聲明本C++函數：

LPCWSTR* MW_ListReaders(_ULONG Context, int* NumberOfReaders);

下給了我「錯誤的DLL調用約定」。

Private Declare Function ListReaders Lib "MyDLL.dll" (ByVal Context As Long, ByRef NumberOfReaders As Integer) As Long

Answer 1

有在C++聲明沒有指定調用約定。大多數C/C++編譯器默認爲__cdecl。如果函數實際使用__cdecl，那麼你將不能夠調用它VB6：

How To Call C Functions That Use the _cdecl Calling Convention

It is not possible to directly call a C function in a DLL if that function uses the _cdecl calling convention. This is because Visual Basic uses the _stdcall calling convention for calling functions. This is a problem because if _cdecl is used, the calling function is responsible for cleaning up the stack. However, if _stdcall is used, the called function is responsible for cleaning up the stack.

NOTE: An .EXE file created in Visual Basic will allow you to call a DLL function that has been declared with the _cdecl calling convention without an error. It is only when you try to call such a function when running a program from the Visual Basic IDE, that Visual Basic generates the following error:

Run-time Error '49': Bad DLL Calling Convention

The fact that the EXE version allows you to call such functions has been confirmed to be a bug by Microsoft. You should not rely on this behavior as this might change in future versions of Visual Basic.

Answer 2

除了雷米的回答，你也得到了VB的聲明稍有不當：

Private Declare Function ListReaders Lib "MyDLL.dll" (ByVal Context As Long, ByRef NumberOfReaders As Long) As Long 

「Integer」是vb中的2字節整數。