When using the comparison operator with a null int, how does it work?

Problem

I’m learning about nullable types and came into the following behavior.

When I try nullable int, I notice that the comparison operator produces an unexpected result. For example, the output of my code below is “both and 1 are equal.” It’s worth noting that it doesn’t print “null” as well.

int? a = null;
int? b = 1;

if (a < b)
    Console.WriteLine("{0} is bigger than {1}", b, a);
else if (a > b)
    Console.WriteLine("{0} is bigger than {1}", a, b);
else
    Console.WriteLine("both {0} and {1} are equal", a, b);

Am I missing something here? I was hoping that any non-negative integer would be bigger than null.

Asked by Ron5504

Solution #1

It’s down the page in the “Operators” section, according to MSDN:

Because an is null, both a > b and a b evaluate to false…

Answered by nkvu

Solution #2

As MSDN says

int? num1 = 10;
int? num2 = null;
if (num1 >= num2)
{
    Console.WriteLine("num1 is greater than or equal to num2");
}
else
{
    // This clause is selected, but num1 is not less than num2.
    Console.WriteLine("num1 >= num2 returned false (but num1 < num2 also is false)");
}

if (num1 < num2)
{
    Console.WriteLine("num1 is less than num2");
}
else
{
    // The else clause is selected again, but num1 is not greater than 
    // or equal to num2.
    Console.WriteLine("num1 < num2 returned false (but num1 >= num2 also is false)");
}

if (num1 != num2)
{
    // This comparison is true, num1 and num2 are not equal.
    Console.WriteLine("Finally, num1 != num2 returns true!");
}

// Change the value of num1, so that both num1 and num2 are null.
num1 = null;
if (num1 == num2)
{
    // The equality comparison returns true when both operands are null.
    Console.WriteLine("num1 == num2 returns true when the value of each is null");
}

/* Output:
 * num1 >= num2 returned false (but num1 < num2 also is false)
 * num1 < num2 returned false (but num1 >= num2 also is false)
 * Finally, num1 != num2 returns true!
 * num1 == num2 returns true when the value of each is null
 */

Answered by Parimal Raj

Solution #3

To summarize, every inequality comparison with null (>=,,=, >), even if both operands are null, returns false. i.e.

null >  anyValue //false
null <= null     //false

With null (==,!=), any equality or non-equality comparison works ‘as intended.’

null == null     //true
null != null     //false
null == nonNull  //false
null != nonNull  //true

Answered by GDS

Solution #4

When comparing C# to SQL, there are a few things to keep in mind.

a=null and b=null in C# = a==b = true

a=null and b=null => a==b => false SQL: a=null and b=null

Answered by Gunnar Siréus

Post is based on https://stackoverflow.com/questions/15777745/how-does-comparison-operator-works-with-null-int