passing byte[] from native ios (plugin) to c# Unity?

I have this native code in ios plugin:

 int _mergeImage(const unsigned char* imageBackground, 
           int backgroundArraySize, int backgroundWidth, int backgroundHeight, 
           const unsigned char* imageForeground, int foregroundArraySize, 
           int foregroundTopLeftX, int foregroundTopLeftY, 
           int foregroundWidth, int foregroundHeight, 
           void* dataPtr){
 CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, imageBackground, backgroundArraySize, NULL);
 CGImageRef backImageRef = CGImageCreateWithPNGDataProvider(provider, NULL, false, kCGRenderingIntentDefault);
 
 CGDataProviderRef foregroundProvider = CGDataProviderCreateWithData(NULL, imageForeground, foregroundArraySize, NULL);
 CGImageRef foreImageRef = CGImageCreateWithPNGDataProvider(foregroundProvider, NULL, false, kCGRenderingIntentDefault);
 
 UIImage* uiImageBack = [UIImage imageWithCGImage:backImageRef];
 UIImage* uiImageFore = [UIImage imageWithCGImage:foreImageRef];
 
 UIGraphicsBeginImageContext(CGSizeMake(backgroundWidth, backgroundHeight));
 
 [uiImageBack drawInRect:CGRectMake(0, 0, backgroundWidth, backgroundHeight)];
 [uiImageFore drawInRect:CGRectMake(foregroundTopLeftX, foregroundTopLeftY, foregroundWidth, foregroundHeight)];
 UIImage *result = UIGraphicsGetImageFromCurrentImageContext();
 UIGraphicsEndImageContext();

 NSData* data = UIImagePNGRepresentation(result);
 NSLog(@"Background array size:%d and data.length:%d", backgroundArraySize, data.length);
 
 dataPtr = data.bytes;
     NSLog(@"Done copying...Background array size:%d and data.length:%d", backgroundArraySize, data.length);
 return data.length;

}

Then I have this code in Unity side(in C#):

 [DllImport("__Internal")]
     private static extern  int _mergeImage (byte[] imageBackground, int backgroundArraySize, 
  int backgroundWidth, int backgroundHeight,
  byte[] imageForeground, int foregroundArraySize, 
         int foregroundTopLeftX, int foregroundTopLeftY, 
  int foregroundWidth, int foregroundHeight, out IntPtr buff);
         
     private static byte[] MergeImage (byte[] imageBackground, int backgroundArraySize, 
                                 int backgroundWidth, int backgroundHeight, byte[] imageForeground, int foregroundArraySize, 
                                 int foregroundTopLeftX, int foregroundTopLeftY, int foregroundWidth, int foregroundHeight)
     {        
             Debug.Log ("Starting....");
             IntPtr unmanagedPtr = IntPtr.Zero;
             Debug.Log ("Calling");
             int length = _mergeImage (imageBackground, backgroundArraySize, backgroundWidth, backgroundHeight, imageForeground, 
                              foregroundArraySize, foregroundTopLeftX, foregroundTopLeftY, foregroundWidth, foregroundHeight, out unmanagedPtr);
             Debug.Log ("Done with length :" + length);    
             byte[] result = new byte[length];
             Debug.Log ("Copying.....");
             Marshal.Copy (unmanagedPtr, result, 0, length);
             Debug.Log ("Want to free it");
             Marshal.FreeHGlobal (unmanagedPtr);
             Debug.Log ("Done.....");
             return result;
     }

I get this error(with previous print out) when i debug console:

 Copying.....
 ArgumentNullException: Argument cannot be null.
 Parameter name: src
 at System.Runtime.InteropServices.Marshal.Copy (IntPtr source, System.Byte[] destination, Int32 startIndex, Int32 length) [0x00000] in <filename unknown>:0     
 at ImageUtilIOS.MergeImage (System.Byte[] imageBackground, Int32 backgroundArraySize, Int32 backgroundWidth, Int32 backgroundHeight, System.Byte[] imageForeground, Int32 foregroundArraySize, Int32 foregroundTopLeftX, Int32 foregroundTopLeftY, Int32 foregroundWidth, Int32 foregroundHeight) [0x00000] in <filename unknown>:0 
 at ImageUtilIOS.MergeImage (System.Byte[] imageBackgroundPNG, Vector2 backgroundDimension, System.Byte[] imageForegroundPNG, Vector4 foregroundBound) [0x00000] in <filename unknown>:0 

The key point is: I want to passing byte[] of the processed image in native code to c#