Nullable Types(?) and Null Coalescing Operator(??)

1.Nullable Types '?' : 

In C-sharp Types are divided into 2 categories

Value Types: Int, float, double, struct, enums etc
Reference Types: Interface, class, delegates, arrays etc

Nullable type is to provide null value to any type.

By Default value types are non-nullable to make them nullable use "?"

Exp:

int i = 0;              // i is non nullable because data type of i is int, so i cannot set to null .
int i = null;         // it will generate compiler error

so we use "?" to provide null value to 'i'

Syntax:
int? i = null;

Exp:

1. {  
2.      static void Main()  
3.      {  
4.           string name = null;  
5.           int i= null;  
6.           int? i = null;  
7.      }  
8. } 

In Line 4 we initialize name to null since it is legal because string is the reference type variable.
In Line 5 we initialize i to null it will generate a compiler error because i is Value type. So, we use "?" in line 6 to make it legal and nullable.

2.Null Coalescing Operator '??' :

This is used to define a default value for nullable types or reference type it return the left hand operand if the operand is not null otherwise it return the right hand operand.

Syntax:

              x=y??1;

Exp:

1. class Test
2. {
3.    static void Main()
4.    {
5.       int availableTickets;
6.       int? ticketOnSale = null;
7. 
   
8.          if(ticketOnSale == null)
9.          {
10.             availableTickets = 0;  // Assign a value when it satisfy null condition
11. 
             }
12. 
    
13.          else
14.          {
15.             availableTickets = (int)ticketOnSale;
             }
16. 
    
17.          Console.WriteLine(availableTickets);
       }
    } 

OR

1. class Test
2. {
3.    static void Main()
4.    {
5.       int availableTickets;
6.       int? ticketOnSale = null;
7. 
   
8.        availableTickets = ticketOnSale??0;      // null-coalescing operator              
9. 
   
10.       Console.WriteLine(availableTickets);
       }
    }