Incorrect "Use of unassigned local variable" for (x is T y) == false.

[NightElfik]

Version Used:
Microsoft Visual Studio Community 2017
Version 15.2 (26430.16) Release
VisualStudio.15.Release/15.2.0+26430.16
Microsoft .NET Framework
Version 4.6.01586

Steps to Reproduce:
This produces "Use of unassigned local variable" but it should not.

if ((variable is Type x) == false) {
    return;
}
x.Function();

This should be logically equivalent to code above and it works fine.

if (!(variable is Type x)) {
    return;
}
x.Function();

We like to use == flase instead of ! in our code base for added readability. Unfortunately this does not work with above statement :/ Would it be possible to add it?

Thanks!

[sharwell]

We like to use == flase instead of ! in our code base for added readability.

📝 Note that for typical C# developers, the use of == false implies that the left hand side is the nullable type bool? and expressly not of type bool. For example, the following code would need to use an explicit comparison:

static void SomeExtension<T>(this List<T> list)
{
  if (list?.Any() != true)
  {
    // The list is null or empty
    return;
  }

  // do something here...
}

[HaloFour]

As this proposal includes a change to the syntax of the C# language I'd suggest opening it on the C# Language Repository.

Oh nevermind, I assumed that there was a request to allow == false to negate the pattern, not that it already works but trips up definite assignment analysis.

@sharwell

I do know a number of C# developers who prefer using == false to negate Boolean expressions as it stands out more than a !.

[gafter]

The C# ! operator inverts "definitely assigned when true" and "definitely assigned when false". The use of == false does not do that. According to the rules of definite assignment, the compiler is correct.

[CyrusNajmabadi]

We like to use == flase instead of ! in our code base for added readability. Unfortunately this does not work with above statement

@NightElfik Very interesting case. As Neal points out, this is currently by design as per the C# language, and dotnet/roslyn is for bugs on the C# compiler.

:/ Would it be possible to add it?

Ah! Quite possibly. For us to do that, we'd need a proposal filed over at https://github.com/dotnet/csharplang . Basically, i think we want a proposal that says that definite assignment should take into account boolean expressions involving == and != when compared to a constant boolean value.

[NightElfik]

Thanks @CyrusNajmabadi, I have opened a new proposal at dotnet/csharplang.