How is null represented in .NET

Question

I was having a conversation with a co-worker and the subject of null came up. He was telling me that in .NET behind the scenes it is just a really small number. I always tho

Answer 1

When a nullable type instance is set to null, its underlying value is zero.

To be more specific, a nullable type is a structure that combines a value of the underlying type together with a boolean null indicator. An instance of a nullable type has two public read-only properties: HasValue of type bool, and Value of the nullable type’s underlying type.

HasValue is true for a non-null instance and false for a null instance.

So when HasValue is true, the Value property returns the contained value. When HasValue is false, an attempt to access the Value property throws an exception. But if you could access it, you would find it contains zero.

Answer 2

From MSDN:

The null keyword is a literal that represents a null reference, one that does not refer to any object

To try and make it a little clearer in the older versions of c#, a value type couldn't be null, ie, it HAD to have a value, if you didn't assign one, you got a potentially random value so something like:

int i;
i++;
console.writeline(i);

in the older versions of c# objects had to be initialized otherwise they were null, which meant they had no reference to any object.

Now with nullable value types in c# 2.0+ you can have a nullable int which means that if you have this code:

int? i;
i++;
console.writeline(i);

you will actually get an exception at i++ because i has never been initialized to anything other than null. If null was 0, this code would execute fine because it'd just evaluate to 0+1, however this is incorrect behaviour.

If null was always 0 and you had a a nullable int, and you wrote some code like:

int? i;
if (int == 0)
{
//do something
}

there is a very real possibility you could get some unexpected behaviour IF null was the same as 0 because there is no way the compiler could differentiate between the int being null and the int being explicitly set to 0.

Another example that clarifies things in my mind:

public int? AddNumbers(int? x, int? y)
{
    if (x == null || y == null)
        return null;
    if (x == 0)
        return y;
    if (y == 0)
        return x;

    return x + y;
}

in this example, it's clear that null and 0 are very different because if you were to pass in 0 for x or y, and null was equal to 0, then the code above would never get to the checks for x == 0 or y == 0, however if you run the code and pass in 0 for x or y, the checks do get executed.

Answer 3

from my experience, I believe Nothing will work, and so will DBNull.Value

Answer 4

It's 0.

Snip from ldnull in ECMA-335 spec:

It might be thought that ldnull is redundant: why not use ldc.i4.0 or ldc.i8.0 instead? The answer is that ldnull provides a size-agnostic null – analogous to an ldc.i instruction, which does not exist. However, even if CIL were to include an ldc.i instruction it would still benefit verification algorithms to retain the ldnull instruction because it makes type tracking easier.