VB6声明C++函数提供了“错误的DLL调用约定”

Question

什么是正确的VB6声明本C++函数：

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

下给了我“错误的DLL调用约定”。

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

Answer 1

有在C++声明没有指定调用约定。大多数C/C++编译器默认为__cdecl。如果函数实际使用__cdecl，那么你将不能够调用它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的声明稍有不当：

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

“Integer”是vb中的2字节整数。