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How big is null in Nullable types?

Posted by Vadim on July 28, 2008

I use often Nullable types in my code.  However, when today I was debugging my code I was surprised to learn that the expression bellow equals to false:

   1: int? n1 = null;
   2: int? n2 = null;
   3: if (n1 <= n2)
   4:     Console.WriteLine("not going to happen");

I decided to dig deeper to see what the reason for this behavior.

I learned that when you have a Nullable type variable that is null (int? n1 = null) in relation comparison expression, the expression is always false.  By relation comparison I mean binary comparison with one of following operators: <, >, <= , =>.

Let me try to explain why this happens.   Let assume that we have two Nullable integers n1 and n2.  n1 is null and n2 has value of 5.  When you compare (n1 < n2), behind the scene the following comparison happens:

(
    (n1.GetValueOrDefault() < n2.GetValueOrDefault()) && 
    (n1.HasValue & n2.HasValue)
)

n1.GetValueOrDefault() will return 0.

n2.GetValueOrDefault() will return 5.

That means the first part of our expression will be true because 0 is less than 5.  However the second part of our expression will be translated into (false & true) which equals to false.  So the first expression is true and the second one is false that means that the whole expression is false too.

That explains why any relation comparison with null value will be false.

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One Response to “How big is null in Nullable types?”

  1. Jeff Brown said

    Huh!
    Interesting semantics. Little weird for greater-than-or-equal and less-than-or-equal though. In many cases I can imagine some arbitrary total ordering would be preferable instead…

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