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Updated an existing Web Site project type Visual Studio 2015, I changed the Framework to 4.6.

I then expected to have all those new features available in my code behind files.

Unfortunately I'm getting errors like:

Error CS8026: Feature 'expression-bodied property' is not available in C# 5. Please use language version 6 or greater.

or e.g.:

Error CS8026: Feature 'interpolated strings' is not available in C# 5. Please use language version 6 or greater.

I did a quick Google check and found a guy posting some comments in a blog posting of ScottGu (search for "8026" on the page).

Since I do not understand his solution, plus I want to have the solution more visible, I've created this SO posting.

My question:

How can I have a Visual Studio 2015 project of type Web Site (i.e. not Web Application) to recognize C# 6 features?

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5
  • 1
    Have you tried adding thw config elements he mentioned in the comment?
    – scheien
    Commented Jul 21, 2015 at 20:15
  • I'm about to do that, but it looks really strange to me, so I was expecting some more clean solution.
    – Uwe Keim
    Commented Jul 21, 2015 at 20:16
  • 5
    Of course, the other option is to stop using those web site "projects", and use Web Application Projects instead.
    – John Saunders
    Commented Jul 21, 2015 at 20:17
  • 2
    @JohnSaunders I would love to do that. Since the migration is a manual task, I fear the time it will take and all the errors I will introduce during the migration. Did it a few times for smaller projects. Always was a hassle...
    – Uwe Keim
    Commented Jul 21, 2015 at 20:33
  • 1
    Glad you've considered it. The comment was mostly for other readers who might think that a "web site" project sounds like what they want.
    – John Saunders
    Commented Jul 21, 2015 at 20:42

8 Answers 8

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144

I've tested this with ASP.NET MVC 5 (tested 5.2.3), and your mileage may vary with other web frameworks, but you just need to add the NuGet package for Roslyn CodeDOM.

Microsoft.CodeDom.Providers.DotNetCompilerPlatform should add the DLL files...

PM> Install-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform

Replacement CodeDOM providers that use the new .NET Compiler Platform ("Roslyn") compiler as a service APIs. This provides support for new language features in systems using CodeDOM (e.g. ASP.NET runtime compilation) as well as improving the compilation performance of these systems.

...and also add the following to your web.config:

<system.codedom>
  <compilers>
    <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701" />
    <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+" />
  </compilers>
</system.codedom>

If the XML is still missing, try adding yourself.

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15
  • 1
    This works as expected. C# 6 now available. Drawback is that my App_GlobalResources/Resources.resx do not generate the auto-class to strongly-typed access the resources anymore.
    – Uwe Keim
    Commented Jul 21, 2015 at 20:30
  • 1
    As a note, I was able to build without errors after project target was changed to 4.6. However, I needed to install this package to resolve red squigglies showing up from intellisense.
    – Doug Morrow
    Commented Oct 8, 2015 at 23:56
  • 8
    I'm using this in a WebForms WebSite (no project file). Sadly, simple .aspx changes trigger a painfully slower recompilation when re-requesting the page in a browser. Maybe Roslyn is theoretically faster for a WebApplication (with proj file), but for WebSites it seems to be significantly slower via csc in the App_Code folder... Anyone seeing the same thing?
    – Stefan Anghel
    Commented Feb 5, 2016 at 17:55
  • 5
    Note, for WebForms to compile with C#6 you need to install NuGet packages Microsoft.CodeDom.Providers.DotNetCompilerPlatform and its dependency Microsoft.Net.Compilers
    – user3638471
    Commented Feb 24, 2016 at 9:42
  • 3
    Note that if you have more than one web site in your solution - things wont work, the easiest way to reproduce is create empty solution and start adding projects into it, after second web site - you will start receive "cannot find csc.exe" errors
    – mac
    Commented Feb 28, 2016 at 7:51
97

See also this blog post from Sayed Ibrahim Hashimi on how to do this through the VS IDE.

In Visual Studio 2015 Update 1 we have included a new feature to simplify this. When you have a solution open which has at least one web project which is targetting .NET 4.5+ and does not have the DotNetCompilerPlatform NuGet package in the Project menu you’ll see a new option, Enable C# 6 / VB 14 appear.
enter image description here

Update.

VS 2017 and 2019 have this feature moved to Build -> ASP.NET Compilation. enter image description here

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2
  • 1
    I opened an older folder-based website project in VS2019 and had to File > Save the project before that option would show up.
    – Corgalore
    Commented Jan 3, 2020 at 21:16
  • Not available in website template
    – IrishChieftain
    Commented Dec 7, 2020 at 13:53
18

I installed the DotNetCompilerPlatform as @jbtule suggested, but was still getting the same errors.

PM> Install-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform

I closed the solution, deleted the bin and obj folders, then opened the solution and rebuilt. Now the C# 6 features work.

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0
13

Possible solutions, taken from the comments on ScottGu's blog posting (search for "8026" on the page):

Solution suggestion 1 (David Taylor)

Add these RTM code dom elements to web.config: 

<system.codedom>
    <compilers>
        <compiler language="c#;cs;csharp" extension=".cs"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701"/>
        <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
</system.codedom>

Then add the Roslyn and Microsoft.CodeDom.Providers.*.dll stuff into your BIN directory.

It is easy enough to setup, but just was strange to me that the default template wasn't set up for it if you selected .NET 4.6 when creating a new "Web Site" in VS 2015 RTM.

Solution suggestion 2 (Martin)

Further to David Taylor's comment above, it looks like the system.codedom settings are correct when a Web App is created with the TargetFramework as the default v4.5.2. Changing the TargetFramework to v4.6 appears to modify the compiler settings for CSharp, in a way that causes an issue.

My workarounds was as follows:

  1. File/New/ASP.NET Web Application
  2. Select "Web API" template from ASP.NET 4.5.2 Templates
  3. Take a copy of the system.codedom element (and its contents) in web.config
  4. Using Properties/TargetFramework, set the Target Framework to 4.6
  5. Replace the modified system.codedom element in web.config with the copy taken prior to changing TargetFramework
  6. Hit F5

Home Page should load as expected.

For info, the system.codedom contents immediately after changing TargetFramework to v4.6. was as follows (note use of the Type "Microsoft.CSharp.CSharpCodeProvider"): 

<system.codedom>
    <compilers>
        <compiler language="c#;cs;csharp" extension=".cs" type="Microsoft.CSharp.CSharpCodeProvider, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701">
            <providerOption name="CompilerVersion" value="v4.0"/>
        </compiler>
        <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb" type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
</system.codedom>
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  • 3
    In regards to your solution 2, Microsoft.CSharp.CSharpCodeProvider version 4.0 means it's not using Rosyln, Thus will not work for C# 6.0.
    – jbtule
    Commented Jul 21, 2015 at 20:28
  • 1
    When migrating from an older solution, make sure <LangVersion> in the csproj file is equal to 6. It can cause problems if it's something else.
    – Saul
    Commented Mar 14, 2016 at 16:16
7

This is not for a Web Site Project. This is how you do it for an ASP.NET MVC project.

You can toggle C# 6 in the Visual Studio UI. This is such an easy option, it is worth trying first.

  1. Right click your project and select Properties.
  2. Click the Build tab.
  3. The build tab has an Advanced... button at the very bottom.
  4. This opens up the Advanced Build Settings as shown below. Select C# 6.0.

enter image description here

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6
  • @Manuzor you might be right. This was for a ASP.NET MVC project.
    – Jess
    Commented Aug 11, 2017 at 13:35
  • @Ucho what is wrong? Is it an MVC project? Do you have the Advanced option in the build settings? Are you missing options for C# language version? Did you set the version and not able to use language features?
    – Jess
    Commented Oct 16, 2018 at 15:17
  • @Jess: Ah, is probably working. I did not notice that my error come from IntelliSense not from build.
    – Yarl
    Commented Oct 17, 2018 at 11:15
  • This setting doesn't seem to apply to code in aspx pages, e.g. <% Eval(whatever) %>.
    – Chris
    Commented Sep 27, 2019 at 19:17
  • Didn't work for my MVC project. The dropdown is disabled, and hyperlink underneath says I can't change versions.
    – Michael come lately
    Commented Jan 26, 2022 at 21:39
7

Here's what it looks like in VS2017 with a website project:

enter image description here

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4
  • I appreciate the crop job @Uwe but I don't think I agree with removing the rest of the top nav since context is lost for where this particular menu item should be.
    – b_levitt
    Commented Aug 28, 2018 at 15:46
  • 1
    Latest VS 2017 has moved this to 'Build -> ASP.NET Compilation'. See my updated answer.
    – Vertigo
    Commented Oct 28, 2019 at 6:58
  • For web site project types I tested @b_levitt's answer on VS2019. (1) Make sure you have a web.config. If not add one. (2) Save the solution by selecting it on the Solution Explorer and issuing [Ctrl] + [S]. (3) Select the project on the Solution Explorer and change the Target Framework to 4.7.2 (4) Select "Enable latest C#..." option from menu Build > ASP.NET Compilation.
    – Denny Jacob
    Commented Jan 1, 2020 at 21:47
  • "The path is not of a legal form"
    – IrishChieftain
    Commented Dec 7, 2020 at 14:11
3

I received this error after making ZERO changes to my WebAPI project. I uninstalled the DotNetCompilerPlatform nuget package and reinstalled, which fixed the problem.

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3

I stumbled across this searching for solution in VS2019 Visual Studio 2019.

Here it is when you have the website project selected in your solution file. "Build → ASP.NET Compilation → Enable latest C# and VB language features".

enter image description here

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1
  • This is a duplicate of other answers; please provide more information on why this differs from those.
    – Mark Schultheiss
    Commented Nov 15, 2022 at 21:20

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