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Posted 28 Sep 2007

The Null Coalescing Operator (??)

, 28 Sep 2007 CPOL
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One of the most useful yet little-known features to come out of C# 2.0


I'm constantly surprised by the number of developers who aren't aware of this handy piece of syntax. It's my favourite thing to come out of C# 2.0 and no developer should be without it.

Like the conditional (?:) operator's big brother... introducing it for your coding pleasure...

The Null Coalescing Operator (??)

The null-coalescing-operator is a brilliant new terse operator that provides syntax for beautifully concise if statements. Essentially, it returns the left-hand-side of the ?? operator, unless null, in which case it executes and returns the right-hand-side of the operator. This may be a statement or a variable reference. Let's jump straight to some examples:

// used inline outputs the value foo or if null returns Undefined
Console.WriteLine("The value of foo is " + (foo ?? "Undefined") + ".");
Input:  foo = "24";
Output: The value of foo is 24.
Input:  foo = null;
Output: The value of foo is Undefined.

The operator is right-associative meaning statements can be chained together; thus returning the first non-null instance:

// assigns foo to the first non-null instance, else returns Undefined
string foo = foo1 ?? foo2 ?? foo3 ?? foo4 ?? "Undefined";
Console.WriteLine("The value of foo is " + foo + ".");

Input:  foo1 = null;
        foo2 = null;
        foo3 = null;
        foo4 = null;
Output: The value of foo is Undefined.
Input:  foo1 = null;
        foo2 = "foo2";
        foo3 = null;
        foo4 = "foo4";
Output: The value of foo is foo2.

Handling null ViewState references:

// try to assign ViewState value as an int, else if null assign 123
int foo = (int?)ViewState["foo"] ?? 123;
Response.Write("The value of foo is " + foo + ".");
Input:  ViewState["foo"]=1;
Output:  The value of foo is 1.  
Input:  ViewState["foo"]=null;
Output:  The value of foo is 123.  

And my personal favorite, on demand field instantiation:

private IList<string> foo;

public IList<string> Foo
        return foo ?? (foo = new List<string>());

Here's an interesting example derived from an idea in the discussions below. It shows how an operator override can be used within an object's definition to enable shorthand syntax for double-null checking. The scenario is checking an object property for null using a null-coalescing-operator, but also defaulting when null-object-reference occurs; which would normally cause a runtime exception. (Note that I don't recommend actually using this approach, I just thought it made an interesting example.)

public class Address
    private static Address Empty = new Address();
    public string StreetName = null;
    public static Address operator +(Address address)
        return address ?? Empty
Console.WriteLine("The street name is "+ (+address).StreetName ?? "n/a" + ".");
Input:  address = new Address();
Output:  The street name is n/a.  
Input:  address = new Address();
            address.StreetName = "Regent St";
Output:  The street name is Regent St.    
Input:  address = null;
Output:  The street name is n/a.

The Rules

To use the null-coalescing-operator, there are some compile-time ground rules.

  • The left-hand-side must evaluate to a reference or nullable type.
  • All evaluated statements must be of matching type, unless they can be implicitly converted.


As you can see from the examples above, this little gem is very powerful and the possibilities are endless. Of course the benefits are purely syntactical, but it helps keep the code clean and easier to follow. I hope you enjoy it as much as I do.


  • 28th September, 2007: Initial post


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


About the Author

United Kingdom United Kingdom
Mike Carlisle - Technical Architect with over 20 years experience in a wide range of technologies.


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Comments and Discussions

GeneralAn alternate view Pin
Big Al 071-Oct-07 6:38
memberBig Al 071-Oct-07 6:38 
GeneralRe: An alternate view Pin
TheCodeKing1-Oct-07 8:18
memberTheCodeKing1-Oct-07 8:18 
GeneralRe: An alternate view Pin
Hal Angseesing1-Oct-07 9:06
memberHal Angseesing1-Oct-07 9:06 
GeneralRe: An alternate view Pin
Big Al 071-Oct-07 14:47
memberBig Al 071-Oct-07 14:47 
GeneralRe: An alternate view Pin
kschulz1-Oct-07 12:08
memberkschulz1-Oct-07 12:08 
GeneralRe: An alternate view Pin
Matware1-Oct-07 16:24
memberMatware1-Oct-07 16:24 
I've got to agree, I came across this operator back when the c# 2.0 language spec was released and went wow, that's as good as the "?:;" operator which I almost never use.
It’s a great operator, but is someone maintaining my code has to dive off to the language reference to fix a bug or add a feature I've failed as a developer.

As developers we have the heavy burden of writing code that performs a function (compiles to a working program) and clearly tells the story of what we were trying to achieve (without the need for comments, as they will always be derivative). This feature doesn't add anything to the earlier and reduces the quality of the later.

As for the issue of more code introduces errors, I think this is one off those off the cuff comments that has become so common that it hast lost all meaning. It refers to the complexity of the code as a function of the complexity of the problem, not size of your CS files as such. For example, writing a recursive XML node walker with hard coded element names instead of using XPath has a lot higher scope for error and will require more maintenance. While writing MissileControl.TestLaunchAllMissiles() instead MC.Go(false) has a lot less scope for error and someone unfamiliar with the code will have a good sense of what it does.

GeneralRe: An alternate view Pin
greenblob3-Oct-07 5:12
membergreenblob3-Oct-07 5:12 
GeneralRe: An alternate view Pin
reinux18-Oct-07 19:06
memberreinux18-Oct-07 19:06 
GeneralRe: An alternate view Pin
peterchen5-Jan-08 10:25
memberpeterchen5-Jan-08 10:25 

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