Why can I only access static members from a static function?

Question

I have a static function in a class.

whenever I try to use non static data member, I get following compile error.

An object reference is required for the nonstatic field, method, or property member

Why is it behaving like that?

Answer 1

A non-static member belongs to an instance. It's meaningless without somehow resolving which instance of a class you are talking about. In a static context, you don't have an instance, that's why you can't access a non-static member without explicitly mentioning an object reference.

In fact, you can access a non-static member in a static context by specifying the object reference explicitly:

class HelloWorld {
   int i;
   public HelloWorld(int i) { this.i = i; }
   public static void Print(HelloWorld instance) {
      Console.WriteLine(instance.i);
   }
}

var test = new HelloWorld(1);
var test2 = new HelloWorld(2);
HelloWorld.Print(test);

Without explicitly referring to the instance in the Print method, how would it know it should print 1 and not 2?

Answer 2

Instance methods rely on state of that particular instance in order to run.

Let's say you had this class which has the scenario you describe:

class Person
{
    static PrintName()
    {
        // Not legal, but let's say it is for now.
        Console.WriteLine(Name);
    }

    private Name { get; set; }
}

Hopefully, the problem is apparent now. Because Name is an instance member, you need an actual instance of the class, since Name can be different across different instances.

Because of this, the static method, which is not attached to an instance, doesn't know which instance to use. You have to be explicit in specifying which one.