[MCI]

Originally Posted by kassane

HANDLE __stdcall MyHandle(void)
{
  return AppHandler;
}

to:

extern "C" __declspec(dllexport) HANDLE MyHandle(void)
{
  return AppHandler;
}

My Example: using [Inno-Setup 5.5.1 + DLL(MSVC 2017 x86)] Works!

//MyDll.cpp

#pragma once

#ifdef MYDLL_EXPORTS
#define MYDLL __declspec(dllexport) 
#else
#define MYDLL __declspec(dllimport) 
#endif

typedef long int HWND;

extern "C" __declspec(dllexport) HWND Func(void)
{
	return 30*5;
}

not working, same problem occurs