Ошибка при вызове loadlibrary не найден указанный модуль

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Пытаюсь в приложении на C# Unity3D вызвать функции из неуправляемой библиотеки. Добавил ее в Assets, со следующими настройками

В коде объявляю их с помощью PInvoke, вроде

[DllImport("avcodec-58.dll"), SuppressUnmanagedCodeSecurity]
public static extern int avcodec_close(AVCodecContext pAVCodecContext);

Но при обращении к ним выдается ошибка «DllNotFoundException: avcodec-58.dll». Подскажите, пожалуйста, как правильно подключить?

Сделал так:

[DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true, CallingConvention = CallingConvention.FastCall)]     
[ResourceExposure(ResourceScope.Process)]
public static extern IntPtr LoadLibrary(string libFilename);

public NativeMethods()
{
    IntPtr lib = LoadLibrary("Полный путь к avcodec-58.dll");
    if (lib == IntPtr.Zero)
        throw new Win32Exception(Marshal.GetLastWin32Error());
}

Но при вызове LoadLibrary все равно выдает ошибку «Не найден указанный модуль».

I have an application that runs as a WCF webservice on IIS7. It calls a COM control and this control then loads another dll with data. This application has been in use for quite a while and normally works fine, but recently I ran into a problem on one machine where the LoadLibrary call to load the data dll fails. However, the call doesn’t always fail, it will work for the first call after the IIS service is restarted, and then fails after that.

The data dll exists in the same directory as the COM control, and the current directory is set to this directory. I’ve checked and the current directory does not get set to another directory, it stays the same for both calls. The separate calls are separate calls to the webservice and in between them the com control gets unloaded, as does the data dll.

The data dll loads fine if I place it in the system directory or use the full path to the dll in the com directory, so I can work around this, but I’m just wondering why it would happen. When I call GetLastError after it fails to load the module, as I mentioned it just gives me ‘The specified module could not be found’ error.

The MSDN description for LoadLIbrary says that the current directory should be searched and all the other installations where this has worked seems to suggest that it normally does. The machine that it’s failing on is a Windows Server 2008 R2 machine, although the application has been run on this type of machine many times before.

Any ideas are welcome. Is there a way to get LoadLibrary to enumerate where it’s looking?

Thanks,
Jordan