I need to switch off the light from a command from the pc. Is this possible using c#?
Does microcontroller recognize c# code?
According to the way you asked the question, it seams that you are not much familiar with writing microcontrollers software. Although .NET Micro Framework exists for them and C# is supported, the general concept of microcontroller programming is quite different compared to desktop application programming. Unless you are willing to spend some time to study microcontrollers, I would recommend you to use some USB relay board instead and control relay directly from your desktop C# application where you feel like home.
You can take a look at this board which has C# examples, or take a look at this SOF topic where similar topic was discussed.