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Posted 18 Jan 2022

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An Oddity of the ?? Operator (C#)

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20 Jan 2022CPOL2 min read
The ?? operator is odd when combined with exceptions - It helps assignments but not non-assignments. Why?
This is a "quick complaint" of a feature that I consider very annoying which Visual Studio is trying to make us use, when the feature isn't really making code more readable, it is just making it "more compact" in just half the situations.

This is how we would probably do things in old C# code:

if (input == null)
  throw new ArgumentNullException("input");

_input = input;

Now, we can reduce the entire thing to just:

_input = input ?? throw new ArgumentNullException("input");

The odd thing is that if I am not assigning input to _input, the old code will just lose code while the new code needs to go back to the old code, becoming:

if (input == null)
  throw new ArgumentNullException("input");

With the new ?? operator, we can't just remove the _input = . That will simply not compile.

Isn't that odd?

Why can't just input ?? throw new ArgumentNullException("input"); mean the same as...

if (input == null)
  throw new ArgumentNullException("input");

... when we are not assigning anything to a variable?

I understand it would make no sense for non-assigments if we were just using ?? to read a variable, but when there is an exception being thrown or a method being called, why not?

It should work as an "if replacement" in those circumstances too, don't you think?

Expressions vs Statements - Does it matter?

I got an answer that the reason ?? works in one case and not the other is because it is an operator and only works as part of expressions, not statements.

Although there is some truth in there, the ?. is also an operator and works on bare statements. So, assuming throw is usually a statement, operator ?? should possibly work with it too.

Also I see that even in MSDN (for example this one about async enumerators) they compare what we write, and what the code is really compiled into (so, we can say that one expression is just a synctactic sugar for another).

That's why I will do this comparison:

obj ?? ObjIsNull(); // Makes sense
obj ?? throw new Exception(); // Makes sense
obj ?? x; // Doesn't make any sense.

var y = obj ?? x; // This one works right now.

All of those can actully be seen as synctactic sugar for:

if (obj == null)
  ObjIsNull(); // Should compile fine.

if (obj == null)
  throw new Exception(); // Should compile fine.

if (obj == null)
  x; // Should cause a compile-time error.

var y = obj;
if (y == null)
  y = x; // Should also work fine.

So, I believe that independenty if we can justify why it doesn't work in statements, the operator ?? is perfectly applicable in statements when we have method calls or when throwing exceptions.

This is Not an Article

Just to make it obvious, this is not an article, that's why I didn't put introduction, background and things like that. Also, I will not discuss coding styles, like using { and } on every if (which actually makes the presented problem even worse) or using the nameof() keyword.

This is just a "quick complaint" of a feature that I consider very annoying that Visual Studio is trying to make us use, when the feature isn't really making code more readable, it is just making it "more compact" in just half the situations.


  • 20th January, 2022: Added Expression vs Statements topic
  • 19th January, 2022: Initial version


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

Written By
Software Developer (Senior) Microsoft
United States United States
I started to program computers when I was 11 years old, as a hobbyist, programming in AMOS Basic and Blitz Basic for Amiga.
At 12 I had my first try with assembler, but it was too difficult at the time. Then, in the same year, I learned C and, after learning C, I was finally able to learn assembler (for Motorola 680x0).
Not sure, but probably between 12 and 13, I started to learn C++. I always programmed "in an object oriented way", but using function pointers instead of virtual methods.

At 15 I started to learn Pascal at school and to use Delphi. At 16 I started my first internship (using Delphi). At 18 I started to work professionally using C++ and since then I've developed my programming skills as a professional developer in C++ and C#, generally creating libraries that help other developers do their work easier, faster and with less errors.

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Codeproject MVP 2012, 2015 & 2016
Microsoft MVP 2013-2014 (in October 2014 I started working at Microsoft, so I can't be a Microsoft MVP anymore).

Comments and Discussions

GeneralRe: The answer is... Pin
wkempf19-Jan-22 10:29
wkempf19-Jan-22 10:29 
GeneralRe: The answer is... Pin
Paulo Zemek19-Jan-22 11:01
Paulo Zemek19-Jan-22 11:01 
GeneralRe: The answer is... Pin
wkempf20-Jan-22 2:38
wkempf20-Jan-22 2:38 
GeneralMaybe you should ask this as a question instead? Pin
PIEBALDconsult19-Jan-22 2:31
professionalPIEBALDconsult19-Jan-22 2:31 
GeneralRe: Maybe you should ask this as a question instead? Pin
Paulo Zemek19-Jan-22 7:50
Paulo Zemek19-Jan-22 7:50 
I am using C# online for quick tests... using C# Online Compiler | .NET Fiddle with .NET 6 actually gives me support for that expression. And it works... it seems that in any place a value is expected (a a null is possible), be it on assignments or when invoking a method, we can use the ?? operator.
GeneralRe: Maybe you should ask this as a question instead? Pin
wkempf19-Jan-22 9:36
wkempf19-Jan-22 9:36 
QuestionDiscard variable Pin
Uroš Šmon18-Jan-22 20:10
professionalUroš Šmon18-Jan-22 20:10 
AnswerRe: Discard variable Pin
Paulo Zemek18-Jan-22 20:14
Paulo Zemek18-Jan-22 20:14 

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