Csharp/CSharp Tutorial/Data Type/Nullable

A nullable type

<source lang="csharp">using System;

class MainClass{

 public static void Main() { 
   int? count = null; 

   if(count.HasValue) 
     Console.WriteLine("count has this value: " + count.Value); 
   else  
     Console.WriteLine("count has no value"); 

   count = 100; 

   if(count.HasValue) 
     Console.WriteLine("count has this value: " + count.Value); 
   else  
     Console.WriteLine("count has no value");     
 } 

}</source>

count has no value
count has this value: 100

Assign value to nullable int

<source lang="csharp">using System;

class MainClass {

 public static void Main() { 
   int? count = null; 
   int? result = null; 

   int incr = 10; 

   count = 100; 
   result = count + incr; 

   if(result.HasValue) 
     Console.WriteLine("result has this value: " + result.Value); 
   else  
     Console.WriteLine("result has no value");     

 } 

}</source>

result has this value: 110

Nullable long

<source lang="csharp">using System; public class Employee {

   public Employee( string Name ) {
       this.firstName = firstName;
       this.terminationDate = null;
       this.ssn = default(Nullable<long>);
   }
   public string firstName;
   public Nullable<DateTime> terminationDate;
   public long? ssn; 

} public class MainClass {

   static void Main() {
       Employee emp = new Employee( "A");
       emp.ssn = 1234567890;
       Console.WriteLine( "{0} {1}", emp.firstName);
       if( emp.terminationDate.HasValue ) {
           Console.WriteLine( "Start Date: {0}", emp.terminationDate );
       }
       long tempSSN = emp.ssn ?? -1;
       Console.WriteLine( "SSN: {0}", tempSSN );
   }

}</source>

Nullable Struct

<source lang="csharp">using System; struct Point {

  public int x;                                   
  public int y;                                   
  public Point(int xVal, int yVal)             
  {
     x = xVal; 
     y = yVal;
  }

} class MainClass {

  static void Main()
  {
     Point p = new Point(6, 11);     
     Point? nullablePoint  = new Point(5, 10);     
     Console.WriteLine("p.x: {0}", p.x);
     Console.WriteLine("p.y: {0}", p.y);
     Console.WriteLine("nullablePoint.x: {0}", nullablePoint.Value.x);
     Console.WriteLine("nullablePoint.y: {0}", nullablePoint.Value.y);
  }

}</source>

p.x: 6
p.y: 11
nullablePoint.x: 5
nullablePoint.y: 10

Nullable Types Access: Explicitly use properties

<source lang="csharp">using System; class MainClass {

  static void Main()
  {
     int? nullableInteger = 15;
    
     if (nullableInteger.HasValue)
        Console.WriteLine("{0}", nullableInteger.Value);
  }

}</source>

15

Nullable Types Access: shortcut syntax

<source lang="csharp">using System; class MainClass {

  static void Main()
  {
     int? nullableInteger = 15;
    
     if (nullableInteger != null)
        Console.WriteLine("{0}", nullableInteger);
  }

}</source>

15

Nullable Types Assignment

<source lang="csharp">using System; class MainClass {

  static void Main()
  {
     int? nullableInteger1, nullableInteger2, nullableInteger3;
     nullableInteger1 = 28;
     nullableInteger2 = nullableInteger1;
     nullableInteger3 = null;
     Console.WriteLine("nullableInteger1: {0}, nullableInteger2: {1}", nullableInteger1, nullableInteger2);
     Console.WriteLine("nullableInteger3 {0} null", nullableInteger3 == null ? "is" : "is not");
  }

}</source>

nullableInteger1: 28, nullableInteger2: 28
nullableInteger3 is null

Null Coalescing Operator

<source lang="csharp">using System; class MainClass {

  static void Main()
  {
     int? nullableInteger = null;
     Console.WriteLine("nullableInteger: {0}", nullableInteger ?? -1);
     nullableInteger = 10;
     Console.WriteLine("nullableInteger: {0}", nullableInteger ?? -1);
  }

}</source>

nullableInteger: -1
nullableInteger: 10

null unification

<source lang="csharp">using System; using System.Collections.Generic; using System.Reflection; using System.Runtime.InteropServices; public class MainClass {

  public static void Main(){
       int? x1 = null;
       int? x2 = new int?();
       int? y1 = 55;
       int? y2 = new int?(55);
       Console.WriteLine("{0}...{1}...{2}...{3}", x1, x2, y1, y2);
       Console.WriteLine("x1 == null? {0}", x1 == null);
       object x1o = x1; 
       Console.WriteLine("x1o == null? {0}", x1o == null);
       Console.WriteLine("{0}...{1}", x1.GetType(), x1o.GetType());
  }

}</source>

......55...55
x1 == null? True
x1o == null? True
Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an ob
ject.
   at MainClass.Main()

Use nullable objects in expressions: result contains null, because one operand is null

<source lang="csharp">using System;

class MainClass {

 public static void Main() { 
   int? count = null; 
   int? result = null; 

   int incr = 10; 

   result = count + incr; 

   if(result.HasValue) 
     Console.WriteLine("result has this value: " + result.Value); 
   else  
     Console.WriteLine("result has no value");     

 } 

}</source>

result has no value

Using ??

<source lang="csharp">using System;

class MainClass {

 static double myMethod() { 
     Console.WriteLine("In myMethod()."); 
     return 0.0; 
 } 

 public static void Main() { 
   double? defaultValue = 1.5; 
   double currentBalance; 

   currentBalance = defaultValue ?? myMethod(); 
   Console.WriteLine(currentBalance); 
   
   defaultValue = null; 
   currentBalance = defaultValue ?? myMethod(); 
   Console.WriteLine(currentBalance); 
   
 } 

}</source>

1.5
In myMethod().
0