I don't have a lot of experience with Java (and none with C#), but a couple of years ago I did have occasion to code some numerics with sparse matrices in Java

I don't have a lot of experience with Java (and none with C#), but a couple of years ago I did have occasion to code some numerics with sparse matrices in Java (because Neo4j), I ended up using http://la4j.org/ http://la4j.org/ which was OK, but nowhere near the scope of numpy (and it seems to have been abandoned since). Possibly other/better options are available now ...