Thank you to everyone for your thoughts. It appears to be unrelated to the version of .Net. I found the source for MemoryStream and the Dispose method does NOT set the buffer reference to null. I tried assigning a static buffer to the MemoryStream instance instead of allowing it to allocate its own buffer but that caused a crash. I created test WPF and WinForms apps and the results are bizarre. We have a small C# class - VideoOCRWrapper - which drives an attached OCR document scanner. It essentially makes calls to a C++ DLL, and has a callback which receives a IntPtr containing a HWND. On receipt of a bitmap, it converts it to a BitmapImage and fires an event to provide a client with the BitmapImage isntance. I created a WinForms app, which initialises VideoOCRWrapper and displays the received bitmap. It has converts the BitmapImage to a Bitmap for display.It shows no memory leaks. I created a WPF app, which initialises VideoOCRWrapper and displays the received bitmap. It shows a huge memory leak consistent with the bitmap buffer leaking. The two apps use almost the same code, except that the WinForms overrides the form's WndProc, whereas the WPF app hooks its own WndProc onto the main window. I memory profiled the WPF test app, and it said that "22 types have instances that are queued for finalization. This can indicate that a Finalizer method is stuck, which will prevent instances from being finalized and cause memory leaks.". There is an extra factor. We use a third party DLL for document verification, which receives bitmaps from us. If I remove that DLL, our code shows no leaks. If I add that DLL, we have leaks but only when running our WPF test app, or our WPF application. When running the WinForms test app, we have no leaks.
