C# out of memory after calling a memory-hungry native-code DLL (Delphi)

Question

I have a C# application that does a few bits and pieces, but the main task it performs is done by a Delphi DLL which it calls.

This Delphi DLL is a total memory hog, which needs to cache a lot of DB-information locally for speed. I'm happy that it's not leaky, as FastMM4 isn't reporting any memory leaks when the code is run within Delphi.

I am starting to run into problems, however, when the control is returned to C#. The C# code attempts to do some calculations on the results of the Delphi app (all results marshalled via a DB). These calculations usually involve a million or so doubles so not extreme memory usage, however the code keeps returning me out of memory exceptions.

I assume that FastMM4 in the Delphi code still hasn't returned the freed memory to Windows (and hence available to the C# code), so the process is still using it's maximum 32-bit memory allocation and C# can't obtain more when it needs to.

So, how do I get the memory used (and freed) by Delphi usable again by the C# code? I thought we may want to do one of the following:

• Force an unload of the Delphi DLL from the C# side (my colleague doesn't think this will work, as he thinks it'll just unload the code rather than the memory used on the heap) - probably LoadLibrary/FreeLibrary?
• Make a call at the end of the Delphi DLL to release the memory back to Windows (I tried SetWorkingProcessSetSize before, but didn't seem to do anything, should I use a different call?)
• Wrap the Delphi DLL in a C# DLL and call it in a different AppDomain (I don't like this from a style perspective as we're creating wrappers just to hold wrappers.
• Anything else I've missed?

Answer 1

Force an unload of the Delphi DLL from the C# side (my colleague doesn't think this will work, as he thinks it'll just unload the code rather than the memory used on the heap) - probably LoadLibrary/FreeLibrary?

This will just work. When the DLL unloads, FastMM will finalize and return the memory that it reserved and committed.