Performance gains in re-writing C# code in C/C++

One option is to rewrite the C code as unsafe C#, which ought to have roughly the same performance and won't incur any interop penalties.

First, profile your code. You might find some real headsmacker that speeds the C# code up greatly.

Second, writing the code in C using pointers is not really a fair comparison. If you are going to use pointers why not write it in assembly language and get real performance? (Not really, just reductio ad absurdam.) A better comparison for native code would be to use std::string. That way you still get a lot of help from the string class and C++ exception-safety.

Given that you have to read 2-3 million records from the DB to do this work, I very much doubt that the time spent cracking the strings is going to outweigh the elapsed time taken to load the data from the DB. So, consider instead how to structure your code so that you can begin string processing while the DB load is in progress.

If you use a SqlDataReader (say) to load the rows sequentially, it should be possible to batch up N rows as fast as possible and hand off to a separate thread for the post-processing that is your current headache and reason for this question. If you are on .Net 4.0 this is simplest to do using Task Parallel Library, and System.Collections.Concurrent could also be useful for collation of results between the threads.

This approach should mean that neither the DB latency nor the string processing is a show-stopping bottleneck, because they happen in parallel. This applies even if you are on a single-processor machine because your app can process strings while it's waiting for the next batch of data to come back from the DB over the network. If you find string processing is the slowest, use more threads (ie. Tasks) for that. If the DB is the bottleneck, then you have to look at external means to improve its performance - DB hardware or schema, network infrastructure. If you need some results in hand before processing more data, TPL allows dependencies to be created between Tasks and the coordinating thread.

My point is that I doubt it's worth the pain of re-engineering the entire app in native C or whatever. There are lots of ways to skin this cat.

There's no reason to write in C over C++, and C/C++ does not exist.

The performance implications of marshalling are fairly simple. If you have to marshal every string individually, then your performance is gonna suck. If you can marshal all ten million strings in one call, then marshalling isn't gonna make any difference at all. P/Invoke is not the fastest operation in the world but if you only invoke it a few times, it's not really gonna matter.

It might be easier to re-write your core application in C++ and then use C++/CLI to merge it with the C# database end.