C# and C++ are two completely different languages. C++ is derived from C whereas C# is more close to JAVA.
so the short answer is that it is not mandatory to learn C# or even C to learn C++. But if you want to learn both then it is completely your choice
better to learn C first. then you can move to C# without learning C++. but learning some basics of C++ helps to understand the concepts of C# easier than who directly starts C# as thier first language.