Razor & null propagation - not working under explicit C# 6 MVC 5 project

Question

Current project:

ASP.NET 4.6.2
MVC 5
Visual Studio 2015 Community v14.0.25431.01 Update 3
Installed the CodeDOM Providers for .NET Compiler nuget package via Project -> Install C# 6
Confirmed that I do have the compiler element in my Web.Config and that it is referencing C# 6

My Razor code:

@if(Session["Type"]?.ToString() == "Insurance") {
  <text>policy of insurance</text>
} else if(Session["Type"]?.ToString() == "Warranty") {
  <text>policy of warranty</text>
} else {
  <text>protection policy</text>
}

Visual Studio explicitly flags this with an error,

Feature 'null propagating operator' is not available in C# 5. Please use language version 6 or greater.

And Visual Studio is explicitly using C# 6:

When I try to run the page itself, I get an error:

CS1525: Invalid expression term '.'

which shows that it is explicitly going after the period following the question mark.

This project was explicitly started as a C# 6 project, and I am quite confused as to why it is derping back to C#5.

Another strange issue: this is working just fine both in debug as well as on my local test setup (I publish to the filesystem, view using local IIS), but when I upload to the server (which has many, many other C#6 sites) it craps out. This is the first razor-side quirk I have ever run across that actually throws an error this badly.

NB: The correct operator for null-coalescing is `??`, and `?.` is null-conditional operator. Since CodeDOM provider for Roslyn used there, have you either activated Copy Local option for related assemblies or setting target framework properly? — Tetsuya Yamamoto, Jun 07 '17 at 02:05
Sorry, it was late in the day, I was tired and had a headache. You are correct, I am trying to make use of the null-conditional operator. Copy Local has been set to True for `Microsoft.CodeDom.Providers.DotNetCompilerPlatform`. The target framework was set to DotNet 4.6.2 when I first created the solution. All projects in the solution are using the same framework. — René Kåbis, Jun 07 '17 at 18:05
@VMAtm you will have to be a bit more clear. I have specified C# 6 in the Advanced Build window of my project properties, and I have explicitly upgraded the project itself to C#6 via the `Project -> Install C#6` menu option. C#6 usage by the compiler has been confirmed via the compiler element in the `web.config`. What else should I be doing? — René Kåbis, Jun 09 '17 at 21:57
I meant that you have to update either `DotNetCompilersPlatform` and `Compilers` packages and set your project to `c# 7.0`. Related: https://stackoverflow.com/a/44384196/213550 — VMAtm, Jun 12 '17 at 15:23
Could it be that in more recent VS/.net-versions explicit targeting has become more or less redundant? (https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/configure-language-version) — qqtf, Aug 10 '22 at 08:46

score 11 · Answer 1 · edited Oct 13 '20 at 15:32

I was having a similar problem and found that I was missing the following from my Web.config file:

<system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:default /nowarn:1659;1699;1701" />
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.8.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:default /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
    </compilers>
</system.codedom>

Razor & null propagation - not working under explicit C# 6 MVC 5 project

1 Answers1