What is (! is)?

Question

I have a similar code:

if (variable !is AnyType)
{
    // TODO
}

But what is that !is? It compiles only in C# preview and Rider doesn't show any errors.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/is#testing-for-type-compatibility — ΦXocę 웃 Пepeúpa ツ, Jun 12 '19 at 13:54
To do "if not is", you would have to do `if (!(variable is AnyType))` — Lasse V. Karlsen, Jun 12 '19 at 13:55
But my JetBrains Rider tells me it's completely fine. :thinking: okay — misticos, Jun 12 '19 at 13:55
[tag:fubo] I asked what is !is and whether it's correct rather than how to negate the is. — misticos, Jun 12 '19 at 13:56
I will try to compile it rn, cuz JetBrains tells it's all ok — misticos, Jun 12 '19 at 13:56
but [tag:fubo] i dont need the valid positive and negative. You misunderstand. lol. I need to know what is !is and why is it correct (at least JB tells me so) — misticos, Jun 12 '19 at 13:58
It **isn't** correct. Now, the *syntax highlighter* in Rider doesn't seem to indicate that this is invalid code, but the compiler does, and it *is* invalid. — Lasse V. Karlsen, Jun 12 '19 at 13:58
@IvMisticos this should not compile. And JB Rider should be throwing an error if this is in a .cs file. If it's not then something is wrong with your environment. — DCCoder, Jun 12 '19 at 13:59
no its all fine with my environment idk why is it so, will try to compile and see — misticos, Jun 12 '19 at 13:59
No, he's actually right in that Rider does not *indicate* that this is invalid code. I just tested it. It doesn't compile though and it *is* invalid. This seems to be a bug in the C# parser used for the syntax highlighting. — Lasse V. Karlsen, Jun 12 '19 at 14:00
@IvMisticos your code is just wrong and there doesn't exit something like `!is` it doesn'T even compile — fubo, Jun 12 '19 at 14:01
@LasseVågsætherKarlsen I just tested it myself, that is odd. Seems to just be a bug in Rider/Resharper — DCCoder, Jun 12 '19 at 14:01
The feature 'nullable reference types' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version. — misticos, Jun 12 '19 at 14:02
So in fact i guess it is valid but only for preview and it is not negative for is. — misticos, Jun 12 '19 at 14:02
Just because I can't resist... it depends on what your definition of `is` is. — EJoshuaS - Stand with Ukraine, Jun 12 '19 at 14:17

Lasse V. Karlsen · Accepted Answer · 2019-06-12T14:12:06.043

In your comments you say this is Rider not telling you that it is invalid.

However, it is invalid code. You will not be able to compile C# code using this construct.

However, it also seems that Rider doesn't indicate that this is invalid code:

It should add "squigglies" beneath the code, like in this example:

So this seems to be a bug in the parser that handles syntax highlighting and validation.

Regardless of this, the code is invalid, and does not compile:

~~Judging from lv Misticos' (now deleted) answer, this may be legal code in C# 8 preview, but right now it is invalid.~~

OK, now it became clear.

In C# 8 preview, the ! is not associated with the is operator, it is associated with the variable, so what you have is understood as this:

if (variable! is AnyType)

Let me rephrase that in pseudo-code:

if (variable /* that isn't null, honestly, just trust me*/ is AnyType)

So yes, this is in fact going to be legal C# code if all plans for C# 8 come to fruition.

I guess JetBrains have added support for syntax highlighting of C# 8 things ahead of time. I still say it is some form of bug in Rider though, they shouldn't allow future C# syntax until it is actually supported. For instance, I do not have C# 8 or any such preview installed so the latest C# Rider is able to use on my PC is 7.3 (I think).

What is (! is)?

1 Answers1