"out var" scope

[yaakov-h]

Is there an intentional difference in the scope of an out var-declared variable depending on the structure it is defined in?

I'm seeing a strange difference where if I declare a variable in if (Foo(out var thing)) then the scope leaks, but if it is declared in as while (Foo(out var thing)) then the scope does not leak.

i.e.:

async Task DoWork()
{
    while (!queue.TryTake(out var nextFile))
    {
        await Task.Delay(TimeSpan.FromMilliseconds(100));
    }

    MyBigBallOf(nextFile); // error CS0103: The name 'nextFile' does not exist in the current context
}

Sharplab playground here.

I would very much like for this to work without an error.

[HaloFour]

Yes, pattern variables and out declarations within a while loop condition do not leak to the outer scope. This was an explicit decision based on the propensity that a capturing lambda within the loop body would enclose over the single leaked variable scope and not the value of the variable within the loop. That was a common source of confusion and bugs with capturing in foreach loops to the extent that the team was willing to accept a breaking change to the language to resolve, so they didn't want to repeat that mistake.

https://github.com/dotnet/csharplang/blob/master/meetings/2016/LDM-2016-11-30.md#scope-of-while-condition-expression-variables
dotnet/roslyn#15529