You can use C# for pretty much anything the question is how convenient it is to do so. There are even some people who try to use .NET with embedded systems. I would use C++ only in cases where performance is the most important aspect of the product and even that niche grows smaller and smaller as time marches on. For example, at my current job we are using C# to implement some pretty intense image processing algorithms. Initially we wanted to use C++ for the performance critical parts, but we are still well within our performance targets using C# alone.