Don’t you have a feeling that sometimes C# code goes too much in null-checking? Yep, me too. But there are few C# 6 null-conditional operators that change it for good.
Null-conditional / Elvis operator – ?.
Let’s start with null-conditional operator – it’s ?.. It’s also called ‘elvis-operator’. Not sure why? Have a closer look – you will see a pair of eyes with elivs like haircut. The operator lets you access members and elements only when the receiver is not-null, returning null result otherwise.
int? length = people?.Length; // null if people is nullThe above is more less equivalent to:
int? length = (people != null) ? (int?)people.Length : null;Except that people variable is only evaluated once. So it’s more like:
var tempVariable = people;
int? length = (tempVariable != null) ? (int?)tempVariable.Length : null;
Second null-conditional operator – ?[]
There is also second null-conditional operator that lets you access elements by index operator.
Person first = people?[0]; // null if people is null
Type personType = people?[0].GetType(); // null if people is nullYou can also chain null-conditional operators.
int? count = customers?[0].Orders?.Count();
Null-coalescing operator – ??
The null-coalescing operator was designed to be used easy with null-conditional operators. It provides default value when the outcome is null.
int length = people?.Length ?? 0; // 0 if people is null
Null-conditional delegate invocation
Delegate invocation can’t immediately follow the ? operator – too many syntactic ambiguities. Following will not work:
if (myDelegate?(args) ?? false) { … } // Won’t compile!However, you can call it via the Invoke method on the delegate:
if (myDelegate?.Invoke(args) ?? false) { … } // Will work fineVery useful way to use it is for event triggering.
PropertyChanged?.Invoke(this, args);Note it’s thread-safe way to check for null as the left-hand side is evaluated only once.
Very Helpfull thanks.
Nice. Just verified that
PropertyChanged?.Invoke(this, new EventArgs(args));
only creates the EventArgs object if PropertyChanged is not null.
quite informative, thank you
Thank you helped me alot
Awesome Post
thank you .
it’s very very useful for me.
What about:
Type personType = people?[0]?.GetType();
It will be equivalent to something like this:
var tempPeople = people; // Get 'people' value only onceif (tempPeople is null) { personType = null; }else{var tempItem = tempPeople[0]; // Get 'people[0]' value only onceif (tempItem is null) { personType = null; }else { personType = tempItem.GetType(); }}In your event triggering example. The null conditional operator isn’t thread safe. Meaning, if another thread unsubscribes from you event between the evaluation and invocation stages, then the invoke will be called on a null value. NullReferenceException.
Hi Micah,
In the documentation, I found information that the variable is evaluated only once between check and invocation. That means if some other thread change the variable it won’t affect the invocation ‘in-progress’.
You can find more info here: https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-6#null-conditional-operators
It’s called the Elvis operator because the Visual Studio team long ago adopted persona names for the VB, C#, and C++ developers (this was circa 2000 for v1). The C# persona was named Elvis. C++ persona was Einstein. and so on
Interesting, thank you for sharing :)
Thank you for your article. I’m fairly new in c# and I’m afraid I don’t understand one thing:
int? length = people?.Length
When should I use Null-conditional / Elvis operator? Is it a good practice to write a code which doesn’t check if people are null and throw loud exception like “Something is wrong because people are null”?
I’ve heard that type of application/program should always be in a valid state – in the state that we expected.
Hi,
In general, it’s good practice to write code that is null-safe. But from experience, I know that many people don’t do it.
Regards,
Mariusz