Memory leak in NSImage conversion when using autoreleasepool

I have a memory leak in my program where I convert a NSImage to a byte array. I tried to dispose of all variables and use an autoreleasepool:

void LoadNewScreenImage()
    {
        byte[] pixels;
        using (NSAutoreleasePool autoreleasepool = new NSAutoreleasePool())
        using (NSImage nsimage = computerInterface.GetComputerScreenForDisplayIndex(0))
        using (CGImage image = nsimage.CGImage)
        using (CGDataProvider dataProvider = image.DataProvider)
        using (NSData data = dataProvider.CopyData())
        {
            pixels = new byte[data.Length];
            System.Runtime.InteropServices.Marshal.Copy(data.Bytes, pixels, 0, Convert.ToInt32(data.Length));
        }
    }

-(NSImage*)getComputerScreenForDisplayIndex:(NSInteger) displayIndex
{
NSImage* computerScreenshot = 0;

CGDisplayCount displayCount=0;

if(self.displays ==  nil)
{

    self.error = CGDisplayNoErr;
    displayCount = 0;

    //allocate enough memory to hold all the display ids that we have
    self.displays = (CGDirectDisplayID *)calloc((size_t)displayCount, sizeof(CGDirectDisplayID));


    //how many displays do we have?
    self.error = CGGetActiveDisplayList(0, NULL, &displayCount);
}

//if there are displays
if(self.error !=  CGDisplayNoErr)
    return 0;


//get the list of active displays
self.error = CGGetActiveDisplayList(displayCount, self.displays, &displayCount);

self.image = CGDisplayCreateImage(self.displays[displayIndex]);

computerScreenshot = [[NSImage alloc] initWithCGImage:self.image size:(NSSize){(CGFloat)CGImageGetWidth(self.image),(CGFloat)(CGImageGetHeight(self.image))}];

CGImageRelease(self.image);
return computerScreenshot;
}

I've tested this native function in a pure objective-c program and I don't get any memory leaks. I am calling LoadNewScreenImage() from the main thread. Does anyone know what is causing this leak or how I could get rid of it? Any help is very much appreciated!