Building a Windows Plugin for Unity: Lessons Learned

Here’s some fun trivia about Cosmic DJ: the way that we achieve our rock-solid musical timing and responsive tap input is by using an external audio engine that I wrote. Generally speaking I would not recommend doing this! However, it’s just not really possible to have perfect metronomic time if you are running your metronome in the main game loop. Fluctuations in frame rate mean fluctuations in the smallest timestep available to the metronome, so if a frame takes longer than a 16th note you’ll hear the rhythm get a little wonky! Anyway, that’s a separate discussion, I’m going to talk about all of the things I got caught on while trying to build this non-trivial plugin.

Develop in Visual Studio 2008

A major thing I got stuck on for a while was trying to build the plugin with Visual Studio 2010. It seemed reasonable enough to do so, but it turns out you must build the plugin against .NET 2.0, which was not something I was even able to select in VS2010. I attempted to install older versions of .NET to build against by searching around the Microsoft download site, but only wound up with installers that didn’t work properly in Windows 7. Visual Studio 2008, meanwhile, defaulted to .NET 2.0, so I used that. Other build settings that seemed to matter a lot were:

  • General > Common Language Runtime support => No Common Language Runtime Support
  • General > Charater Set => Use Multi-Byte Character Set
  • C/C++ > Code Generation > Enable C++ Exceptions => Yes (/EHsc)
  • C/C++ > Code Generation > Runtime Libarary => Multi-threaded (/MT)

Unity has a really simple example plugin in their documentation that I’d recommend simply copying all of the settings out of. It’s called Simplest Plugin and is linked under the Examples section of Build Plugins For Desktop Platforms in the Unity Manual. I suggest looking for this page in your local documentation, just to be safe.

Create A Single Project Solution

There are a couple other libraries included in the plugin that I wanted to statically link to, such as my own partial port of Minim to C++ (YMMV), and like you do I wanted to have a solution that included the vcproj for Minim as a sub-project. This turned out to be mostly problematic because of the very particular build settings required by Unity. I found it difficult to make sure all of my projects agreed in their settings and also linked together properly.

Put DLL Dependencies In The Correct Place

Windows doesn’t have audio file reading routines built-in (like CoreAudio does in OSX for instance), so we leverage a couple open source libraries to make this happen (libsndfile and mpg123 if you must know). Both of these libraries are distributed such that you link against a .lib, but the real code lives in a DLL that is loaded at runtime. Unity can handle this but not quite in the way you’d expect.

To work in the Editor: Any DLLs that your plugin depends on must be put in the top level of your Unity Project folder, not in Assets/Plugins as you might expect.

To work in the Standalone Player: Any DLLs that your plugin depends on must be put in the same folder as the exe file, which either means copying them by hand or writing a post-process script to copy them.

You Can Debug Your DLL Using Visual Studio

A cool thing: it’s possible to debug your plugin code in Visual Studio while the DLL runs in the Unity Editor. For your debug flavor you’ll want Runtime Library set to Multi-threaded debug (/MTd). With your project open in Visual Studio: start the Unity Editor, go to the Tools menu in Visual Studio and choose “Attach To Process…”, choose Unity.exe from the list (Visual Studio might hang out for a bit before becoming responsive again), set some breakpoints, and press Play on a scene in Unity that has a script that calls into your plugin. When you call into code that has a breakpoint, VS will catch it and you can inspect variables and do like you would normally do. Important: Unity won’t load your plugin until the first time a script calls out to it, so when you initially attach to the Unity process it’ll look like your breakpoints are inaccessible, which is true because the DLL hasn’t been pulled into the executable yet.

Use String Marshaling When Returning Strings

Behavior seems inconsistent across platforms with regard to how Mono deals with strings returned from plugin functions. In OSX it seems to work fine to return static strings, but in iOS and Windows this causes a problem when Mono tries to deallocate that memory. If you search around the web you’ll see that some folks recommend allocating a new string and returning that, so that everything is cool when Mono tries to deallocate it, but I was still getting crashes on Windows with that method. Instead, what you want to do is return a const char * from your plugin code and then have this in the C# binding:

Importantly, PtrToStringAnsi copies the string represented by the IntPtr and also expands characters to unicode, so now you’ve got a string that is made from managed memory and will not cause crashes when it goes away. Of course this means you definitely do not want to allocate a new string when you return one to Unity because that memory will be leaked.