Can we write a c# program without a Class or Structure?

NO!
As everybody else said, there must be at least one class that MUST have the Main method, that is the starting point of a C# application and that is what C# compiler looks for when compile the code.

You can emulate global functions by using public static methods but they'd still have to be defined inside a class.

No, it's not possible.

You need at least one class or struct to contain your Main() method as C# doesn't support global functions.

I think we can't because of following reasons

i) C# is fully OOP oriented and it must need at-least a parent class or struct
ii) C# is purely developed to support encapsulation and if you avoid classes then it will not execute

Let's wait for more reply here.