Nullable types in C#

Nullable types in C# can represent all the values of an underlying type, and an additional null value. For example, nullable integer can have values like 10, 333, and null.

Nullable types have also a ?? -operator, that defines a default value that is returned when a nullable type is assigned to a non-nullable type. For example, you can use it to form a following operation: "a = b, unless b is null, in which case a=c". Or you can think it to be similar to SQL's NVL()-function.

Below is a code sample how to use nullable types and ??-operator.

    1 using System;

    2 

    3 namespace NullableTest

    4 {

    5   class Tester

    6   {

    7     static void Main(string[] args)

    8     {

    9       int? i = null;

   10       int? k = 5;

   11 

   12       // d = c, unless c is null, in which case d = -1.

   13       int a = i ?? -1;

   14 

   15       Console.WriteLine("Value of a is {0}", a);      

   16 

   17       // check if i has value

   18       if(i.HasValue)

   19         Console.WriteLine("Value of i is {0}", i.Value);

   20       else

   21         Console.WriteLine("i is null");

   22 

   23       // check if k has value

   24       if (k.HasValue)

   25         Console.WriteLine("Value of k is {0}", k.Value);

   26       else

   27         Console.WriteLine("k is null");

   28 

   29       Console.ReadLine();

   30     }

   31   }

   32 }

Further reading at MSDN:

Using Nullable Types (C# Programming Guide)

Nullable Types (C# Programming Guide)