Notes

using System;

namespace ConsoleApp1
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello World!");

class3 c3 = new class3();
}
}

/* Class types
* 1. regular class
* can be got an instance (creating by new keyword)
* can be inherited (may be base class)
* can interits
*/

class class1
{
//members
}

/* Static class
* can not be got and instance (we cannot create by new keyword)
* can not be inherited
* cannot inherit
* all members of static class must be static
*/

static class class2
{
static int a;

//members
}

/* Sealed class
* cannot be inherited
* only can be created by new keyword
*
*/

sealed class class3
{
//members
}

/* abstract class
* can not be created by new keyword
* must only be inherited
* members may be abstract or regular
*
*/

abstract class class4
{
public class4()
{

}
int a;
void message()
{
Console.WriteLine("Hello");
}

public abstract void Sum(int a, int b);
public abstract string WhoAreYou();
}

class A : class4
{
public override void Sum(int a, int b)
{
Console.WriteLine(a + b);
}

public override string WhoAreYou()
{
return "I am overriden abstract method body";
}

}

abstract class MathHelper
{
public abstract void Sum(int a, int b);
public abstract void Minus(int a, int b);
public abstract void Multiply(int a, int b);
public abstract void Divide(int a, int b);
}

class Math : MathHelper
{
public override void Sum(int a, int b)
{
Console.WriteLine(a + b);
}
public override void Minus(int a, int b)
{
Console.WriteLine(a - b);
}
public override void Multiply(int a, int b)
{
Console.WriteLine(a * b);
}
public override void Divide(int a, int b)
{
Console.WriteLine(a / b);
}
}

}