Object is currently in use somewhere else error when using Paraller.For

Question

I am trying to parallelize an operation on an image using the code below , but I get an One or more errors occurred error.

Looking into the inner exception I can see the error {"Object is currently in use elsewhere."} has occurred.

Note: CoverImage and SecretImage are Bitmaps

 Parallel.For(0, CoverImage.Width, x =>
            {
                for (int y = 0; y < CoverImage.Height; y++, SecretImageHeight++)
                {
                    Color coverImagesPixel = CoverImage.GetPixel(x, y);
                    Color secretImagesPixel = SecretImage.GetPixel(SecretImageWidth, SecretImageHeight);

                    int r = (coverImagesPixel.R & coverImageMask) + ((secretImagesPixel.R >> shift) & secretImageMask);
                    int g = (coverImagesPixel.G & coverImageMask) + ((secretImagesPixel.G >> shift) & secretImageMask);
                    int b = (coverImagesPixel.B & coverImageMask) + ((secretImagesPixel.B >> shift) & secretImageMask);

                    StegoImage.SetPixel(x, y, Color.FromArgb(255, r, g, b));
                }
                SecretImageHeight = 0;
                SecretImageWidth++;
            }
            );

I tried using a mutex like this :

mutex.WaitOne();
StegoImage.SetPixel(x, y, Color.FromArgb(255, r, g, b));
mutex.ReleaseMutex();

But still to no avail (I get the same error).

What types are CoverImage and SecretImage? Are they in use in the application elsewhere? What happens when you wrap access to them in the mutex - they might be causing the problem as well. — thorkia, Jun 18 '15 at 14:08
If you follow the exception, which line causes it? I'm guessing either `SecretImage` or `CoverImage` is not thread-safe. — flindeberg, Jun 18 '15 at 14:09
@thorkia:They are of type Bitmap. @ Flindeberg: It just highlights the Parallel.For block altogether — Hossein, Jun 18 '15 at 14:12
@Hossein Have you tried changing to a non-parallel loop then adding a break point inside the loop and walking through it to see where the exception is being thrown? I suspect Jon Skeet is right and you either have to copy the images to a bytearray or dispatch the access to them on the UI thread — thorkia, Jun 18 '15 at 14:16
possible duplicate of [InvalidOperationException - object is currently in use elsewhere](http://stackoverflow.com/questions/1851292/invalidoperationexception-object-is-currently-in-use-elsewhere) — Henrik, Jun 18 '15 at 14:16

score 2 · Accepted Answer · answered Jun 18 '15 at 14:09

2

It looks like you're trying to use image APIs from non-UI threads. That's a bad idea.

I suggest you copy the image to a byte array on the UI thread, then run your parallel for loop which would mutate the array, then recreate the image afterwards from the byte array.

answered Jun 18 '15 at 14:09

Jon Skeet

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If this is WPF, the other option is the use the dispatcher and invoke the calls on the UI thread – thorkia Jun 18 '15 at 14:17
1

@thorkia: But the OP is deliberately using `Parallel.For` to parallelize the work - putting each image through a UI-thread bottleneck would defeat the point. – Jon Skeet Jun 18 '15 at 14:22

Object is currently in use somewhere else error when using Paraller.For

1 Answers1