Using C# Program to Interpret Keystrokes
The answer is actually, surprisingly, yes! C# requires .NET CLR for execution, I had assumed that .NET exists only in microsoft desktop land, but it has in fact been ported to microcontrollers and released open source as .NET micro, it can even run baremetal with no OS. 

However this is not possible on an 8 bit regular arduino, it requires one of the larger 32 bit models. There is a prebuilt clone that has the .NET micro CLR framework ported to it on an STM32F4 cpu called the netduino

Unfortunately the footprint for .Net Micro is too large to run on a regular 8bit arduino and probably hasn't been built for 8bit systems. It is possible with the arduino Due (in theory) and on other micrcontroller platforms that support .NET micro to run C# code outside of the windows platform.