How to debug c++ dll code

If I used a c/c++ dll as a plugin. How would I step into the code when doing a P/Invoke call. Does MonoDevelop support this?

If not, how could I debug the code in my external dll?

If you’re interested, I found an alternative to writing to text files for debugging unity dll’s in development.

It’s quite simple, just allocate a standard output console using AllocConsole()

FILE * pConsole;
freopen_s(&pConsole, "CONOUT$", "wb", stdout);

Obviously this only works under Windows, and if you hit the close button on the console, it sends the quit message to the main Unity window and kills it.


I have a new method of doing this using a reverse invoke from unmanaged code to managed code.


private delegate void DebugCallback(string message);

private static extern void RegisterDebugCallback(DebugCallback callback);

private void Start()
    RegisterDebugCallback(new DebugCallback(DebugMethod));

private static void DebugMethod(string message)
    Debug.Log("UnmanagedCodeTitle: " + message);


typedef void(__stdcall * DebugCallback) (const char * str);
DebugCallback gDebugCallback;

extern "C" VOID __declspec(dllexport) RegisterDebugCallback(DebugCallback callback)
    if (callback)
        gDebugCallback = callback;

void DebugInUnity(std::string message)
    if (gDebugCallback)

Using Unity Pro, it is possible to step into native C++ code by opening the DLL project in VS and attaching the debugger to the Unity.exe executable. (Attach to process)

As far as I can tell, it is not possible to debug C# using Visual Studio, one has to use MonoDevelop.

Note also that you cannot debug C# using MonoDevelop while debugging a native DLL with Visual Studio. VS will complain about an access violation. You have to pick your poison.

Couldn't say about the step into the code, since I haven't yet had the time to test that point in Unity 3.x.

But before Unity 3.x we still developed dlls to work with Unity. There ARE ways, they're just not as pretty :) Usually I would develop the class in Visual Studio and work only on the connection to Unity - passing variable arrays to and from Unity. This way you can always send debug variables and arrays easily at every step you need. Once that connection is made accessible - it's much easier to work.

Then I continued developing the DLL in C++, test it with Unity, back and forth - each time spending more and more time in Unity, and less and less time in the dll.

At one point we tried opening sockets from the C++ to send text data to Unity, which was listening, just for making debugging easier.

Also - text files are your friend - you can write and append to text files from DLLs. Have a simple script or the unix tool "tail" run from a command prompt window - and show the output of that text file on the screen as it's appended to. live. It's just like using Console.WriteLine - The output updates instantly.

you could debug that using visualstudio in windows, or gdb (or xcode using gdb) on mac. make sure to compile your plugin with debug symbols, start up unity, attach your debugger, place a breakpoint in its entrypoint, and you should be good to go.