C# 7 features don't work within a web project on Visual Studio 2017 RC

Question

I have several projects in the solution, and the C# 7 features, such as tuples and throw expressions, work fine in all of the library projects, but there is a (non Core) web project that doesn't compile due to errors on the C# 7 features. Right after compiling, the error window quickly clears itself, presumably because the IDE/editor compiles the same units without error. I have to use the output window to see the compiler errors. It is as though the IDE/editor are assuming C# 7, but the compiler used in the build is not.

I've tried adding "__DEMO__,__DEMO_EXPERIMENTAL__" to the conditional compilation symbols, to no avail. I've experimented with targeting different version of the framework and have edited the web.config, including the compilation and targetFramework tags of system.web.

Example errors:

if (!config.Properties.TryGetValue(modelId, out var model)) // error CS1003: Syntax error, ',' expected
if (modelDescription is ComplexTypeModelDescription complexTypeModelDescription) // error CS1026: ) expected

Here are the first few lines of the csproj file for the project:

<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.1.0.3\build\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.props" Condition="Exists('..\packages\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.1.0.3\build\Microsoft.CodeDom.Providers.DotNetCompilerPlatform.props')" />
  <Import Project="..\packages\Microsoft.Net.Compilers.1.3.2\build\Microsoft.Net.Compilers.props" Condition="Exists('..\packages\Microsoft.Net.Compilers.1.3.2\build\Microsoft.Net.Compilers.props')" />
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />

Update: I tried creating a new web project using the latest template in VS 2017 RC and copying in my source files. Same thing.

I also tried explicitly setting Project | Properties | Build | Advanced | Language Version to 7. Results in "Invalid option '7' for /langversion".

Answer 1

The solution is to update the Microsoft.Net.Compilers nuget package to >=2.0.0. As of now, for the 2.0.0 version to appear, the "Include prerelease" checkbox at the top of the package manager must be checked. With this installed, the Language version advanced setting doesn't have to be overridden from Default.