Preface

I wondered, what if a class, for example A, has two methods, one that can only be call when A is not a instance, but the other one can only be call when A is an instance.

Pseudo Code

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class A;
A.do1();
A.do2(); // error

a = new A();
a.do1(); // error
a.do2();

I looked up for a while on the Internet, and I found there is actually a thing called Static Member.

What is a static member

There is only one copy of that member.
It is initialized when the program start, even before main starts.
It stands for the entire program life.

For example

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using System;

public class A
{
    public static int do1()
    {
        return 1;
    }
    
    public int do2()
    {
        return 2;
    }
}

public class Program
{
    public static void Main()
    {
        Console.WriteLine(A.do1());
        A a = new A();
        Console.WriteLine(a.do2());
    }
}

But if I do

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a.do1();
// Member 'A.do1' cannot be accessed with an instance reference; qualify it with a type name instead

A.do2();
// An object reference is required for the non-static field, method, or property 'A.do2()'

This is pretty handy when I need to use something that doesn’t belong to the instance.
However, it does belong to the class.

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class Window {
    static HWND getActive() {
        return GetActiveWindow();
    }

    RECT getRect() {
        return this.rect;
    }
}

ActiveWindow is not a part of current window; nevertheless, rect is.

Very interesting. :D