Why is C# CS0136 error not reported when compiled with /langversion:latest

Question

The code below compiles fine in Debug but failed with a CS0136(A local or parameter named 'x' cannot be decaled..) error in Release. Anybody has any idea why the error is not reported in Debug build?

public void test()
{
    Action<int> a = x => x++;
    int x = 0;
}

Target framework 4.6.1
VS version: 16.4.3 and 16.4.5
MSBuildVersion: 16.4.0
MSBuildRuntimeVersion = 4.0.30319

After some investigation, I was able to trace the error to this configuration difference between debug and release build

Debug build has this line in config, but release build does not have.

    <LangVersion>latest</LangVersion>

and this problem can be reproduced with the following two command

csc.exe /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.6.1\System.dll" /langversion:7.3 Program.cs"
csc.exe /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.6.1\System.dll" /langversion:latest Program.cs"

you have 2 variables with the same name. change the second variable name to int y = 0; or anything — Baahubali, Feb 25 '20 at 06:37
@Baahubali I modified my question. I was asking why is the error NOT reported in debug — Xiaoguo Ge, Feb 25 '20 at 06:39
@XiaoguoGe Which version of .NET are you using, is it .NET Core or .NET Framework? I can't reproduce it, observe only _CS0219_ warning in .NET Core. .NET Framework shows _CS0136_ error for debug and release — Pavel Anikhouski, Feb 25 '20 at 08:30
@Pavel Anikhoushi It is a .net framework project targeting 4.6.1 — Xiaoguo Ge, Feb 25 '20 at 22:08

Xiaoguo Ge · Answer 1 · 2020-03-09T05:07:16.060

0

I was able to find some partial answers: This question asked about the same thing. A comment from @Hans Passant referenced This Document which indicates there is a relaxation of the scoping rule for lambdas.

This can explain why /langversion:latest and /langversion:7.3 behave differently

For a .net 4.6.1 project though, I would assume /langversion:latest should result in at most C# 7.3. But apparently it is not the case or maybe there is something else going on here. Hope somebody can shed more light on this.

edited Mar 09 '20 at 05:07

answered Mar 09 '20 at 05:00

Xiaoguo Ge

2,177
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"For a .net 4.6.1 project though, I would assume /langversion:latest should result in at most C# 7.3." - nope, it can still handle *most* C# 8 features. – Jon Skeet Mar 09 '20 at 07:12
@JonSkeet Looks like I misunderstood default language version provided on [this page](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version). Apparently /langversion:latest does not mean default. It overwrites default and uses the C# 8.0 when it is available. – Xiaoguo Ge Mar 09 '20 at 07:25
Yes, exactly - default is only what it says it is: the default if you don't specify anything else. – Jon Skeet Mar 09 '20 at 07:29

Why is C# CS0136 error not reported when compiled with /langversion:latest

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