One of the interesting new features in C# 2.0 was nullable valuetypes. Using these, you can set valuetypes to a value, or null. Their usage is entirely straightforward. For instance, to use a nullable int simply declare a variable of type int?, and then set it to a value or null.<\/p>\n
Behind the scenes, nullable valuetypes are implemented as generic structs of type Nullable<T>. When you write:<\/p>\n
int? i = 2;<\/p>\n
What you’re really writing is:<\/p>\n
Nullable<int> i = new Nullable<int>(2);<\/p>\n
When you assign a value to a pre-existing nullable int, like so:<\/p>\n
i = 4;<\/p>\n
you’re really writing:<\/p>\n
i = new Nullable<int>(4);<\/p>\n
And when you ‘get’ the value, like so:<\/p>\n
int j = i;<\/p>\n
What you’re really writing is:<\/p>\n
int j = i.Value;<\/p>\n
The Value is a get property, and assignment is by creating a new instance of Nullable<T>. This is similar to strings in C#: presenting a reference type by valuetype semantics.<\/p>\n
This is all fine for primitives like int, double, etc. But when using structs, an interesting problem is created that lets light in on the ‘magic’ of nullable valuetypes.<\/p>\n
Let’s say I have a struct Point, implemented as so:<\/p>\n
struct Point<\/p>\n
{<\/p>\n
public int x, y;<\/p>\n
public Point(int px, int py)<\/p>\n
{<\/p>\n
x = px; y = py;<\/p>\n
}<\/p>\n
public override string ToString()<\/p>\n
{<\/p>\n
return “(” + x + “, ” + y + “)”;<\/p>\n
}<\/p>\n
}<\/p>\n
Then, I write a program that creates an int and a Point, and alter them slightly:<\/p>\n
static void Main(string[] args)<\/p>\n
{<\/p>\n
int i = 3;<\/p>\n
Console.WriteLine(++i); \/\/increment then print it<\/p>\n
Point p = new Point(2, 4);<\/p>\n
p.x = 1; Console.WriteLine(p); \/\/move left then print it<\/p>\n
Console.ReadLine();<\/p>\n
}<\/p>\n
No problems here. The int and the Point will be altered as expected. Now I decide that I want my int and Point to be nullable. So I simply add two question marks, producing this program:<\/p>\n
static void Main(string[] args)<\/p>\n
{<\/p>\n
int? i = 3;<\/p>\n
Console.WriteLine(++i); \/\/increment then print it<\/p>\n
Point? p = new Point(2, 4);<\/p>\n
p.x = 1; Console.WriteLine(p); \/\/move left then print it<\/p>\n
Console.ReadLine();<\/p>\n
}<\/p>\n
But this won’t compile. The int? part is fine, but for the Point? part I get the error: “‘System.Nullable< Point>’ does not contain a definition for ‘x'”<\/i>. No problem, I’ll just edit it like so:<\/p>\n
static void Main(string[] args)<\/p>\n
{<\/p>\n
int? i = 3;<\/p>\n
Console.WriteLine(++i); \/\/increment then print it<\/p>\n
Point? p = new Point(2, 4);<\/p>\n
p. Value.x = 1; Console.WriteLine(p); \/\/move left then print it<\/p>\n
Console.ReadLine();<\/p>\n
}<\/p>\n
Now you get a different compile error: “Cannot modify the return value of ‘System.Nullable<.Point>.Value’ because it is not a variable”<\/i>. This happens because Nullable<T>.Value has no set accessor method.<\/p>\n
Nullable value types have this ‘limitation’ for a very good reason. The same thing happens is you try to edit a struct in a List<> of structs. Consider an over-simplification of Nullable<T> with a bodged-in set accessor, like so:<\/p>\n
class MyNullable<T><\/p>\n
{<\/p>\n
private T val;<\/p>\n
public T Value<\/p>\n
{<\/p>\n
set {<\/p>\n
val = value;<\/p>\n
}<\/p>\n
get {<\/p>\n
return val;<\/p>\n
}<\/p>\n
}<\/p>\n
}<\/p>\n
If I create a MyNullable<Point> and use it:<\/p>\n
MyNullable<Point> p = new MyNullable<Point>();<\/p>\n
p.Value = new Point(2, 1);<\/p>\n
p.Value.x = 2;<\/p>\n
I get a compile error on the third line: “Cannot modify the return value of ‘MyNullable<Point>.Value’ because it is not a variable”.<\/i> This is because the statement p.Value.x = 2 is equivilent to:<\/p>\n
Point otherPoint = p.Value; otherPoint.x = 2;<\/p>\n
Which doesn’t change the underlying p.val, because otherPoint is a copy<\/i><\/b> of p.Val. So when C# gives us a compile error, it is merely protecting us from editing an irrelevent copy of the underlying Point.<\/p>\n
In conclusion, nullable valuetypes are immutable. I think the C# designers intended programmers to never modify the value of a struct once one is created, nullable or not.<\/p>\n","protected":false},"excerpt":{"rendered":"
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