C#7 inline As variable scope is too wide

[pixellos]

Versions Used:
2.1.0 (nuget from 24-04-2017),

Release version from
Internal ID: Roslyn-Signed_20170320.4
Internal VS ID: d15rel.26403.00

Steps to Reproduce:

1. new file test.cs with

using System; 

namespace Test
{
    public class Program
    {
        public static void Main()
        {
            Exception e = null;
            if (e is ArgumentException ae)
            {
            }
            if  (e is ArgumentException ae)
            {
            }
        }
    }
}

2. Compile with csc.exe from newest release
   https://github.com/dotnet/roslyn/releases/tag/Visual-Studio-2017-Update-1

Expected Behavior:
Build should have pass.
This construct should be treated like

Exception e = null;
{
    var ae = e as ArgumentException;
    if (ae != null)
    {
    }
}
{
    var ae = e as ArgumentException;
    if (ae != null)
    {
    }
}

Actual Behavior:
Microsoft (R) Visual C# Compiler version 2.1.0.56735
Copyright (C) Microsoft Corporation. All rights reserved.

test.cs(15,17): error CS0128: A local variable or function named 'ae' is already defined in thi
s scope

Code is treated like

Exception e = null;
var ae = e as ArgumentException;
if (ae != null)
{
}
var ae = e as ArgumentException;
if (ae != null)
{
}

[sharwell]

The behavior you are observing is actually by design. It was implemented in order to support common scenarios like this:

if  (!(e is ArgumentException ae))
{
  return;
}

// do something with ae here

📝 Note that I wasn't part of the design for this feature. I'm simply telling you how the outcome of the language design team's work on this feature relates to the behavior you observed.

[sharwell]

I'm leaving this open so one of the language design members can add more context and/or links to related issues to help you understand how to best use this new feature. 😄

[jcouv]

This is indeed by design.
You can find more details in the pattern-matching proposal and issue #12597.

I'll go ahead and close this. Thanks