It is nearly impossible to directly translate C# to C++ so that it will run on Unix machines.
This is mainly due to the fact that the .NET Framework is not available (from C++) on Unix machines. Mono will allow you to run many C#/.NET programs, but does not support C++/CLI (the C++ extensions that allow directly working with the .NET Framework).
Converting the language is possible - though difficult due to differences in approach (e.g., garbage collection in C#), but the framework calls will require porting to different libraries, and it is often not a good candidate for a direct translation.
For example, in your code above, you'd have to decide on a C++ library for web access - and once you had that choice made, it would dictate the code required to call into that library to download the website string.
