?? Operator

The ?? operator, sometimes called the Coalescing Operator, is a binary operator that is part of the syntax for a basic conditional expression in several programming languages, most notably C#.

Conditional assignment

?? is most frequently used to simplify null expressions as follows:

"possibly_null_value" ?? "value_if_null"

The "possibly_null_value" is evaluated "true" or "false" as a Boolean expression. On the basis of the evaluation of the Boolean condition, the entire expression returns "value_if_null" when "possibly_null_value" is null, but "possibly_null_value" otherwise. This is similar to the way ternary operators (?: statements) work in functional programming languages.

This operator's most common usage is to minimize the amount of code used for a simple null check. For example, if we wish to implement some C# code to give a page a default title if none is present, we may use the following statement:

string pageTitle = suppliedTitle ?? "Default Title";

instead of the more verbosestring pageTitle = (suppliedTitle = null) ? "Default Title" : suppliedTitle;orstring pageTitle;

if (suppliedTitle = null) pageTitle = "Default Title";else pageTitle = suppliedTitle;The three forms are equivalent.

JavaScript Implementation

In JavaScript, the equivalent of this is:/* pre: assume that "foo" does not exist
*/

var bar=(document.foo) || "goo";

/* post: "bar" now has a value of "goo".
*/

ee also

*?:
*C#

External links

* [http://blog.devstone.com/Aaron/archive/2006/01/02/1404.aspx New Operator in C# 2.0: ?? (null coalescing operator)]