What ever you were designing, it's almost certain that a more expensive, and mission critical system than yours was already built with C#. Objecting to a major language as C# because it's "closed-source" seems irrational to me.

Why does it matter if it's open or closed-source? It's not like you can realistically invest time to dig in the source code and modify it, in the rare case that you found a problem. C# had huge amounts of money invested on it, probably more than your entire project you were planning.