Allow throw statements on the right hand of the null coalescening assignment operator

[john-h-k]

In C# 8.0, this is valid code:

string x = null;
x = x ?? throw new Exception();"

However, for some reason, this code is not

string x = null;
x ??= throw new Exception

which would be a single line null check, but it generates CS8115 "A throw expression is not allowed in this context".

Suggestion:

Either

• Allow throw expressions on the right side of the null coalescent assignment operator, semantically equivalent to variable = variable ?? throw expression
• Add a new, similar operator or use the null coalescing operator that an expression variable -- operator -- flow change is a valid statement, e.g replacing the x ??= throw ... line above with x ?? throw...

[yaakov-h]

_ = x ?? throw new Exception();

is already valid, if you want a single line.

[DavidArno]

Whilst I can see a reason for making the compiler support x ??= throw new Exception for consistency, to my mind it is an incredibly unclear piece of "clever programming". Even if it were supported, I'd still prefer to write it as:

if (x is null) throw new Exception();

as the intent of the code is then clear. I think this a case of "just because you can [or in theory could] express code in a certain way doesn't mean you should".

[333fred]

I won't say we can't be convinced to change our minds here, but this was a very explicit decision: https://github.com/dotnet/csharplang/blob/c81b99f4c965dbb6e9819a10a2e010db4c859f6f/meetings/2018/LDM-2018-07-16.md#throw-expression-on-the-right-hand-side. Our reasoning basically agreed with @DavidArno's above, we didn't see the benefits over a simple if statement. You're not trying to assign anything at all, you're doing a null check and throwing.

[gafter]

The code

x ??= y;

is not equivalent to

x = x ?? y;

rather it is equivalent to

x ?? x = y;

Which obviously would not permit the throw expression for y.

[bernd5]

good explanation 👍