Ah, but what, you're using both managed code (C#) and unmanaged code (C++) in the same address space, doing memory intensive work using third party proprietary

Ah, but what, you're using both managed code (C#) and unmanaged code (C++) in the same address space, doing memory intensive work using third party proprietary C++ code that does lots of small allocations? In 32-bit land, it's welcome to OutOfMemoryException because managed code can't allocate a big enough chunk when returning to C#. No free lunch :) Either recycle processes often or reserve big enough contiguous chunks in managed code before calling out to the bad C++ code, no amount of good citizenship in managed code will stop the poorly written native code from fragmenting the hell out of your address space.