custom implementation of iTunesMobileDevice.dll throws NullReferenceException

Question

I had intended to implement the Manzana.dll library in order to detect iPhone connection events and interact with the device. The problem is that it only seems to work if the client machine has the iTunes dlls and resources installed, which I cannot rely on. Therefore I am trying to use a custom implementation of the Manzana source code to point it's references to the necessary iTunes files that I am including with the project.

Although everything looks ok the compiled library throws a NullReferenceException when used from my application. The application load and initializes ok, but when an iPhone is connected the connectedevent throws an exception.

The actual error is:

System.TypeInitializationException: The type initializer for 'istreamwrapper.MobileDevice' threw an exception. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at istreamwrapper.MobileDevice..cctor()
--- End of inner exception stack trace ---
at istreamwrapper.MobileDevice.AMDeviceNotificationSubscribe(DeviceNotificationCallback callback, UInt32 unused1, UInt32 unused2, UInt32 unused3, Void*& am_device_notification_ptr)
at istreamwrapper.iPhone.doConstruction()

I was able to use that to narrow down the problem to to this method from my iPhone class

private unsafe void doConstruction()
    {
        void* voidPtr;
        this.dnc = new DeviceNotificationCallback(this.NotifyCallback);
        this.drn1 = new DeviceRestoreNotificationCallback(this.DfuConnectCallback);
        this.drn2 = new DeviceRestoreNotificationCallback(this.RecoveryConnectCallback);
        this.drn3 = new DeviceRestoreNotificationCallback(this.DfuDisconnectCallback);
        this.drn4 = new DeviceRestoreNotificationCallback(this.RecoveryDisconnectCallback);
        int num = MobileDevice.AMDeviceNotificationSubscribe(this.dnc, 0, 0, 0, out voidPtr);
        if (num != 0)
        {
            throw new Exception("AMDeviceNotificationSubscribe failed with error " + num);
        }
        num = MobileDevice.AMRestoreRegisterForDeviceNotifications(this.drn1, this.drn2, this.drn3, this.drn4, 0, null);
        if (num != 0)
        {
            throw new Exception("AMRestoreRegisterForDeviceNotifications failed with error " + num);
        }
        this.current_directory = "/";               
        }
    }

The issue comes from

num = MobileDevice.AMDeviceNotificationSubscribe(this.dnc, 0, 0, 0, out voidPtr);

which points to this code which is located in my MobileDevice class

 [DllImport("iTunesMobileDevice.dll", CallingConvention = CallingConvention.Cdecl)]
    public static unsafe extern int AMDeviceNotificationSubscribe(DeviceNotificationCallback callback, uint unused1, uint unused2, uint unused3, out void* am_device_notification_ptr);

That in turn seems to reference this in it's own class

namespace istreamwrapper
{
[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
internal delegate void DeviceNotificationCallback(ref AMDeviceNotificationCallbackInfo callback_info);
}

which then points to another class with:

namespace istreamwrapper
{
[StructLayout(LayoutKind.Sequential, Pack = 1)]
internal struct AMDeviceNotificationCallbackInfo
  {
    internal unsafe void* dev_ptr;
    public NotificationMessage msg;

    public unsafe void* dev
    {
        get
        {
            return this.dev_ptr;
        }
    }
  }
}

The vast majority of this code was copied straight from the Manzana.dll, the only thing I changed was where the itunesmobiledevice files are located (which is now a set path, rather detected at run time)

Old code:

namespace Manzana
{
  internal class MobileDevice
  {
private static readonly FileInfo iTunesMobileDeviceFile = new FileInfo(Registry.GetValue("HKEY_LOCAL_MACHINE\\SOFTWARE\\Apple Inc.\\Apple Mobile Device Support\\Shared", "iTunesMobileDeviceDLL", (object) "iTunesMobileDevice.dll").ToString());
private static readonly DirectoryInfo ApplicationSupportDirectory = new DirectoryInfo(Registry.GetValue("HKEY_LOCAL_MACHINE\\SOFTWARE\\Apple Inc.\\Apple Application Support", "InstallDir", (object) Environment.CurrentDirectory).ToString());
private const string DLLName = "iTunesMobileDevice.dll";

static MobileDevice()
{
  string str = MobileDevice.iTunesMobileDeviceFile.DirectoryName;
  if (!MobileDevice.iTunesMobileDeviceFile.Exists)
  {
    str = Environment.GetFolderPath(Environment.SpecialFolder.CommonProgramFiles) + "\\Apple\\Mobile Device Support\\bin";
    if (!File.Exists(str + "\\iTunesMobileDevice.dll"))
      str = "C:\\Program Files\\Common Files\\Apple\\Mobile Device Support";
  }
  Environment.SetEnvironmentVariable("Path", string.Join(";", new string[3]
  {
    Environment.GetEnvironmentVariable("Path"),
    str,
    MobileDevice.ApplicationSupportDirectory.FullName
  }));
}

New code:

namespace istreamwrapper
{
class MobileDevice
{
    static MobileDevice()
    {

       string str = "[XX_MYPATHHERE_XX]\\Apple\\Mobile Device Support";
       string AppSuppDirectory = @"[XX_MYPATHHERE_XX]\Apple\Apple Application Support";
       Environment.SetEnvironmentVariable("Path", string.Join(";", new string[3] { Environment.GetEnvironmentVariable("Path"), str, AppSuppDirectory }));

    }

Is there something I'm missing that is causing that call to return null? I'll admit I don't fully understand everything that is happening in the above code so it's entirely possible it's something simple.

Answer 1

Yes, I believe the answer was really that the path was incorrect and thus could not find the file. I just didn't realize this because the error it was throwing was too generic.