Nullable type in C#

In C# Types are divided in 2 Categories:
1. Value Types:-int, float, double,structs, enums etc
2. Reference Types:-array, class, interface, delegate etc

By default value types are non nullable data type. We use ? mark for make them nullable
int a = 0(a is non nullable , so i can't set it to null, it generate error)
int? b=0(b is nullable , so b=null is legal)

Program Using Nullable type :
using System;
class Program
{
    public static void Main()
    {
        int AvailableChocolate;

        int? ChocolateForSale = null;

        if (ChocolateForSale == null)
        {
            AvailableChocolate = 0;
        }
        else
        {
            AvailableChocolate = (int)ChocolateForSale;
        }
        Console.WriteLine("Available Chocolate = {0}",AvailableChocolate);
    }
}