Is there an “opposite” to the null coalescing operator? (…in any language?)

Question

null coalescing translates roughly to return x, unless it is null, in which case return y

I often need return null if x is null, otherwise return x

Answer 1

PowerShell let's you reference properties (but not call methods) on a null reference and it will return null if the instance is null. You can do this at any depth. I had hoped that C# 4's dynamic feature would support this but it does not.

$x = $null
$result = $x.y  # $result is null

$x = New-Object PSObject
$x | Add-Member NoteProperty y 'test'
$result = $x.y  # $result is 'test'

It's not pretty but you could add an extension method that will function the way you describe.

public static TResult SafeGet<T, TResult>(this T obj, Func<T, TResult> selector) {
    if (obj == null) { return default(TResult); }
    else { return selector(obj); }
}

var myClass = new MyClass();
var result = myClass.SafeGet(x=>x.SomeProp);

Answer 2

Haskell has fmap, which in this case I think is equivalent toData.Maybe.map. Haskell is purely functional, so what you are looking for would be

fmap select_y x

If x is Nothing, this returns Nothing. If x is Just object, this returns Just (select_y object). Not as pretty as dot notation, but given that it's a functional language, styles are different.

Answer 3

If you've got a special kind of short-circuit boolean logic, you can do this (javascript example):

return x && x.y;

If x is null, then it won't evaluate x.y.

Answer 4

Delphi has the : (rather than .) operator, which is null-safe.

They were thinking about adding a ?. operator to C# 4.0 to do the same, but that got the chopping block.

In the meantime, there's IfNotNull() which sort of scratches that itch. It's certainly larger than ?. or :, but it does let you compose a chain of operations that won't hork a NullReferenceException at you if one of the members is null.

Answer 5

This is being added in C# vNext (Roslyn powered C#, releases with Visual Studio 2014).

It is called Null propagation and is listed here as complete. https://roslyn.codeplex.com/wikipage?title=Language%20Feature%20Status

It is also listed here as complete: https://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/3990187-add-operator-to-c

Answer 6

The so called "null-conditional operator" has been introduced in C# 6.0 and in Visual Basic 14.
In many situations it can be used as the exact opposite of the null-coalescing operator:

int? length = customers?.Length; // null if customers is null   
Customer first = customers?[0];  // null if customers is null  
int? count = customers?[0]?.Orders?.Count();  // null if customers, the first customer, or Orders is null

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operators