How to use a C++ dll in Unity3D?

Question

I am aware of this similar question , but it does not respond to my problem.

I have written two .dll s using Visual Studio 2010. One is in C++, and communicates with an SDK that was written in C++. The other is a C# wrapper for that C++ library, so that it can be used in C# contexts.

My plan was that this would let me use my code in Unity3D , but apparently that is not the case. It seems like Unity3D does not allow me to import .dll s as Assets if they are not a .NET assembly. So I can add my C# wrapper, but not the C++ dll.

This results in a DllNotFoundException whenever I try to access the C++ library. I have tried simply copying the C++ library into the Assets/Plugins folder, but that gives the same results.

Is there a way to do this properly? This is a very vital part of my project setup.

Answer 1

The problem is that the DLL is not being found when the p/invoke runtime code calls LoadLibrary(YourNativeDllName) .

You could resolve this by making sure that your DLL is on the DLL search path at the point where the first p/invoke call to it is made. For example by calling SetDllDirectory .

The solution that I personally prefer is for your managed code to p/invoke a call to LoadLibrary passing the full absolute path to the native DLL. That way when the subsequent p/invoke induced call to LoadLibrary(YourNativeDllName) is make, your native DLL is already in the process and so will be used.

internal static class NativeMethods
{
    [DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
    internal static extern IntPtr LoadLibrary(
        string lpFileName
    );
}

And then somewhere in your code:

private static IntPtr lib;

....

public static void LoadNativeDll(string FileName)
{
    if (lib != IntPtr.Zero)
    {
        return;
    }

    lib = NativeMethods.LoadLibrary(FileName);
    if (lib == IntPtr.Zero)
    {
        throw new Win32Exception();
    }
}

Just make sure that you call LoadNativeDll passing the full path to the native library, before you call any of the p/invokes to that native library.

Answer 2

Note that the DllNotFoundException can be caused by building your Unity DLL in Debug instead of Release!

A simple oversight that can cause a headache.

Answer 3

This also happens when Unity can find your DLL, but is not able to find it's dependencies. Obvious fix is to place dependency DLLs into /Plugins as well, or link your dependencies statically.

Less obvious reason is when your DLL depends on Visual Studio runtime library dynamically, ie is built with Properties -> C/C++ -> Code Generation -> /MD option. Change it to /MT to link with runtime statically.