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Posted 23 Dec 2010
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The ?? Operator

, 4 Sep 2012
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This operator is introduced to set value in place of null value, it can also be defined in words like 'In case of null, pick value from another'


Operator is introduced with Nullable datatype inclusion in .NET Framework operator ?? can also be referred in words like 'In case of null, pick value from another'.


Suppose you're assigning a value to Nullable bool like:

bool? b = null;
At the time of checking value, it will give you an error like:

if(b) //Error CS0266.

So it's always preferable to use ?? to prevent error like:

if(b ?? false)

It defines that, in case b is null, pick the value false.

?? can also be used in multiple choice of value like:

bool ? a = null
bool ? b = null
bool ? c = true
a = b ?? c ?? false;

That will check b first if b is undefined or null, then it will move further to check for c if that also has null then it will set false to a.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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Hiren solanki
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Comments and Discussions

GeneralMy vote of 1 Pin
afaz4-Sep-12 22:52
memberafaz4-Sep-12 22:52 
GeneralRe: My vote of 1 Pin
Hiren solanki4-Sep-12 23:00
memberHiren solanki4-Sep-12 23:00 
GeneralRe: My vote of 1 Pin
Andreas Gieriet5-Sep-12 3:52
memberAndreas Gieriet5-Sep-12 3:52 
The vote of 1 is a bit too low, but I somehow understand why a lower vote is given.

You not even give the proper name to that operator: it's the Null Coalescing (Wikipedia)[^] operator.

IMHO, the given example is kind of poorly chosen: nullable value types are useful for database type map - otherwise I consider them as a code smell.

A more realistic scenario is in connection with reference types and especially with Enumerable<T> (e.g. linq queries):

Record GetFirstOrSentryRecord(Func<MyRecord, bool> predicate)
    var query = from record in table where record=>predicate(record) select record;
    return query.FirstOrDefault() ?? MyRecord.SomeSentryRecord;

Or with defensive programming, at locations where you want no null values, e.g.:
HandleError(..., context ?? "no context");



The ?? operator is everywhere usable where you have an instance of a reference type (including nullable value types) that may evaluate to null.

expandedwith ternary operatorwith null coalescing operator
result = item;
if (result == null)
    result = other;

result = item != null
       ? item
       : other;

result = item
         ?? other;

Finally, a good tip provides references to say where to go from here, e.g.

GeneralRe: My vote of 1 Pin
Hiren solanki5-Sep-12 3:58
memberHiren solanki5-Sep-12 3:58 
GeneralRe: My vote of 1 Pin
Andreas Gieriet5-Sep-12 5:12
memberAndreas Gieriet5-Sep-12 5:12 

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