C# 0 The Complete Reference

GetZeroBal( )

 is not called because 

balance

 contains a value. 

As explained, when the left-hand expression of 

??

 contains a value, the right-hand expression 

is not evaluated.

Nullable Objects and the Relational and Logical Operators

Nullable objects can be used in relational expressions in just the same way as their 

corresponding non-nullable types. However, there is one additional rule that applies. 

When two nullable objects are compared using the 

<

,

>

,

<=

, or 

>=

 operators, the result 

is false if either of the objects is 

null

. For example, consider this sequence:

byte? lower = 16;

byte? upper = null;

// Here, lower is defined, but upper isn’t.

if(lower < upper) // false

Here, the result of the test for less than is false. However, somewhat counterintuitively, so is 

the inverse comparison:

if(lower > upper) // .. also false!

Thus, when one (or both) of the nullable objects used in a comparison is 

null

, the result of 

that comparison is always false. Thus, 

null

 does not participate in an ordering relationship.

You can test whether a nullable object contains 

null

, however, by using the 

==

 or 

!=

operators. For example, this is a valid test that will result in a true outcome:

if(upper == null) // ...

When a logical expression involves two 

bool?

 objects, the outcome of that expression 

will be one of three values: 

true

,

false

, or 

null

(undefined). Here are the entries that are 

added to the truth table for the 

&

 and 

|

 operators that apply to 

bool?

.

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