c# - Release original ImageSource after a TransformedBitmap is constructed from it -

background:

i'm building application open potentially large number of photos, generate thumbnail present user, allow things exif data viewing/clearing , minor post processing. want allow user scroll through images without pausing load each 1 becomes visible, don't want keep dozens or hundreds of full size bitmap images in memory.

i had built prototype of task using system.drawing using image objects , generatethumbnailimage method, decided move wpf , use system.windows.media.imagesource derived objects , transformedbitmap scaledtransform generate thumbnail.

what found, though, when create transformedbitmap, has reference source image, available , still present in memory. how release source object?

some relevant c# code:

using system.windows.media; using system.windows.media.imaging; using system.io; using system; ... private void loadimage(){     //called class internally handle generating thumbnail     //intent keep metadata , thumbnail bitmap in memory     stream handle = file.openread(filename);     bitmapdecoder source = bitmapdecoder.create(handle,bitmapcreateoptions.none,bitmapcacheoption.onload);     handle.dispose();     //determine scaling ratio force larger of height or width fit inside desired thumbnail size (int)maxdim.     scaleratio = math.min(maxdim/math.max(source.frames[0].pixelheight,source.frames[0].pixelwidth),1); //a public member of class, double     _imagesource = new transformedbitmap(source.frames[0],new scaletransform(scaleratio,scaleratio)); //private member of class, imagesource     _imagesource.freeze();      _exif = source.frames[0].metadata; //private member of class, imagemetadata } 

the problem here while hoped (bitmapdecoder)source released, can still access object via _imagesource.source.

i have considered using copypixels or encoding transformedbitmap byte[] stream create new, unattached bitmap, both of methods seem unnecessary reprocessing if can abandon or dispose of source or if there simple , fast way create shallow clone haven't discovered. attempt @ shallow clone using bitmapframe.create(transformedbitmap) doesn't free memory either, doesn't leave me obvious reference.

some testing watching memory consumption shows each image loaded costs 30mb. approximately 200x200@32bpp image should 160kb, not counting overhead.

the question again tl;dr: how release reference source bitmap after transformedbitmap uses it?