The thing about computer languages is that they come and go.
C and C++ are probably the only languages that will never disappear in our lifetime.
As a beginner, to avoid any unproductive language jumping I would just learn C++ (incl C libraries).
Nothing of any long term value is written in C# anyway. Heck, just look at the recent news about the unpopularity java of and it's security issues and you know the same will happen with .NET soon enough.
With so many trendy hipster developers it is very difficult to argue this point. But do your own research on the topic and I am sure you will come to the same conclusion. It seems like only yesterday it was a tossup between C++ and Visual Basic. Everyone who wrote their software in VB6 at the time pretty much had to do a complete rewrite to get it to work on VB.NET or ultimately port it to C++.
Not to mention that it is always really annoying when some software on Linux/BSD is written using C#. Mono just drags in soo many crap dependencies. Especially since the functionality provided by the .NET framework (such as GUI) is quite weak in comparison to other native offerings.