CODEWITHSUNDEEP
c#.netnullnullablevalue-type
mgamer
mgamer

Reputation: 14060

C# value type initialized with null

In C# 3.0 you can assign null to int? type (in CLR int? is a struct):

int? a = null;

but when you define a custom struct:

struct MyStruct
{

}

there's an error during the compilation of this code:

MyStruct a = null;

The error is as follows:

Cannot convert null to 'Example.MyStruct' because it is a non-nullable value type

While int? is a struct in CLR it is somehow counterintuitive that we can assign null to it. I suppose that null is implicitly casted or boxed to a certian int? value that represents the null value. How is it done precisely? How can I extend MyStruct in such a way that it would be possible to execute the line:

MyStruct a = null;

Upvotes: 7

Views: 5203

Answers (3)

Master Morality
Master Morality

Reputation: 5927

Structs are value types in C#, not reference types, so they can't be null.

MyStruct? = null;

will work, but remember that structs are immutable, as a example:

public struct Point 
{
   public int x, y;

   public Point(int p1, int p2) 
   {
       x = p1;
       y = p2;    
   }
}

void Main()
{

    //create a point
    var point1 = new Point(10, 10); 

    //assign point2 to point, point2
    var point2 = point1; 

    //change the value of point2.x
    point2.x = 20; 

    //you will notice that point1.x does not change
    //this is because point2 is set by value, not reference.
    Console.WriteLine(point1.x);
    Console.WriteLine(point2.x);

}

Upvotes: 2

Jon Skeet
Jon Skeet

Reputation: 1503090

It's because int? is actually shorthand Nullable<int>. You can do the same thing for your type too:

MyStruct? a = null;

null can only be implicitly converted to a nullable type, which means any reference type or any nullable value type - where the latter means Nullable<T> for some T.

Note that this ability to conver from null is really a language feature - the compiler is converting this:

int? a = null;

into

int? a = new int?();

They mean the same thing - basically a "null" value for a nullable value type is a value where HasValue is false, which it will be by default. It's not the same as a null reference - although boxing a null value like that will result in a null reference.

(This is one example of a feature which crosses language, library and CLR boundaries. The library part is quite simple, the CLR part is only to do with boxing and unboxing - but there's quite a lot in terms of lifted operators etc in the language.)

Upvotes: 20

marcelo-ferraz
marcelo-ferraz

Reputation: 3257

just adding, a nullable type can represent the normal range of values for its underlying value type, plus an additional null value.

In notation, just as int is actually Int32, not any int, the ? sign means a nullable. Nullable, in this case.

http://msdn.microsoft.com/en-us/library/1t3y8s4s%28VS.80%29.aspx

Upvotes: 1

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