Why main() function should be static in c#?

If it is not static, it is the instance one, so: what should create the very first instance? Not having any instance in the very beginning is very simple and natural convention. Do you know Occam Razor and why it is important: http://en.wikipedia.org/wiki/Occam's_razor[^]?

—SA

The only reason is because that's what the designers of the language decided to be what to look for as an entry point for your program. They could just as well have used a totally different approach to find the entry point, e.g. using meta data, or instantiating an object for you. The main reason for naming it void main() is that it's intuitive for users coming from other languages, and then making it static makes sense, as described in Abhinavs and SAKs posts.

Because C# (like, for instance, Java) hasn't functions, it has methods. You know a method belongs to an object and you need an instance of the object in order to call it.
So, in principle you couldn't have an entry point in C#, because you haven't pre-built object instances.
The very escape to the above rule are 'static methods': they resembles 'ordinary function' because they don't need an instance of the object to be called.