The ?: conditional operator is commonly used by C# programmers and most developers understand how it works.

This operator is of the form:

condition ? first_expression : second_expression

where if the condition is true, the first expression is evaluated and becomes the result; if false, the second expression is evaluated and becomes the result. This is commonly used as an elegant replacement for if-else blocks.

But… have you run into the ?? operator? This operator is called the null-coalescing operator and can also be used to simplify a special if-else case.

Imagine that you had the following code that was used to display a user’s location… but it might not be set:

...
String locationDisplayString= user.Location;
if(locationDisplayString.Location==null)
{
	locationDisplayString =”not specified”;
}
...

This seems reasonable. If the user has provided a location, then we display it, otherwise we display “not specified” to explain that the user did not want to provide a location.

Could we make this code a bit cleaner ? The ?? operator can be used to evaluate an assignment expression where there is a possibility of the assignment resulting in a null value. It could be described like this:

left_operand ?? right operand

If the left_operand is not null, it will be returned as the result. Otherwise, the right operand will be returned. So.. we could simplify our above code as follows:

...
string locationDisplayString= user.Location ?? “not specified”;
...

Fancy. This operand comes in handy when assigning nullable types to non-nullable types. Generally if you try to assign a nullable type to a non-nullable type you would get a compile-time error.

int? nullableInt=null;
int nonNullableInt=nullableInt;//error. bad.

But, you could take advantage of the ?? operator and set a default value in the event that the nullable int is in fact, null.

int? nullableInt=null;
int nonNullableInt=nullableInt ?? -1;//no error. all good.

You might be thinking… when would I use this?

This could be used in the new ASP.NET MVC Framework controller actions. If your routing is using the default route like "{controller}/{action}/{id}" and you wanted to use an int parameter to represent the id in your action method... what would happen if no id parameter was supplied in the request ?

You could use a nullable int type - and of course you could use the ?? operator to supply a default value and gracefully handle this case.