How could I use C++ code with C#?

Sergio Fonseca

It's an bypass for PostMessageA (For a game) but it's in C++ and I belive C# doesn't have assembler to let me convert it so how could I get this to work with C#? I don't know enought C++ to create DLLs :-(

Code:

DWORD dwPostMessage = (DWORD)GetProcAddress( LoadLibrary( _T("user32.dll") ), "PostMessageA") + 5;
_declspec(dllexport) BOOL WINAPI __PostMessageA( HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam )
{
_asm
{
mov edi,edi
push ebp
mov ebp,esp
jmp dword ptr ds:[dwPostMessage]
}
}

BOOL WINAPI DllMain( HMODULE hModule, DWORD dwReason, LPVOID lpReserved )
{
switch ( dwReason )
{
case DLL_PROCESS_ATTACH:
case DLL_THREAD_ATTACH:
case DLL_THREAD_DETACH:
case DLL_PROCESS_DETACH:
break;
}
}

2 Replies

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Sergio Fonseca18y ago
I'm starting a project in C++ to test if it really bypasses PostMessageA before spending my time on more work. Thank you xP
Alan18y ago
After removing DllMain (which didn't seem to be doing anything) and moving the declaration inside __PostMessageA(), I managed to compile the following code as a C dll (which I called sergio.dll):

#include

_declspec(dllexport) BOOL WINAPI __PostMessageA( HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam )
{
   DWORD dwPostMessage = (DWORD)GetProcAddress( LoadLibraryA( "user32.dll" ), "PostMessageA") + 5;

   _asm
   {
      mov edi,edi
      push ebp
      mov ebp,esp
      jmp dword ptr ds:[dwPostMessage]
   }
}

However, when I tried to call this by changing the P/Invoke signature in the C# program to:

[DllImport("sergio.dll", EntryPoint = "__PostMessageA")]
    static extern bool PostMessage(IntPtr hWnd, uint msg, int wParam, int lParam);

I got a System.AccessViolationException :(

I suspect that this is because the frame pointer register 'ebp' is being modified by the inline assembly code because there was a warning to this effect when I compiled the dll. Your chances of getting this to work from C# are not therefore looking too good.

Of course, there's no way of translating this code to C# because it doesn't support inline assembly of native code or, for that matter, MSIL.