Build .NET Core 2.0 EXE file with C# 7.1

Question

I have a project I'm trying to build. It's using C# 7.1 features, and I can run it via Visual Studio, but when I try to publish to get an .exe file I get the error:

Agent.cs(8,30): error CS8107: Feature 'async main' is not available in C# 7.
Please use language version 7.1 or greater. [C:\Users\stuarts\Documents\Visual
Studio 2017\Projects\Agent\Agent\Agent.csproj]
CSC : error CS5001: Program does not contain a static 'Main' method suitable
for an entry point [C:\Users\stuarts\Documents\Visual Studio
2017\Projects\Agent\Agent\Agent.csproj]

The .csproj (project) file:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>netcoreapp2.0</TargetFramework>
    <IsPackable>false</IsPackable>
    <NetStandardImplicitPackageVersion>1.6.1</NetStandardImplicitPackageVersion>
    <RuntimeFrameworkVersion>2.0.0-*</RuntimeFrameworkVersion>
    <RuntimeIdentifier>win10-x64</RuntimeIdentifier>
    <ApplicationIcon />
    <StartupObject />
  </PropertyGroup>

  <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <LangVersion>7.1</LangVersion>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="RabbitMQ.Client" Version="5.0.1" />
  </ItemGroup>

</Project>

I'm building with:

dotnet publish -c Release -r win10-x64 Agent.csproj

Again, this all works when debugging in Visual Studio. Why is getting a simple .exe file out of a console application project template so awkward!

Answer 1

Your problem is that in the section...

<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
    <LangVersion>7.1</LangVersion>
</PropertyGroup>

...you specify to use C# 7.1 in the Debug configuration.

However, with...

dotnet publish -c Release -r win10-x64 Agent.csproj

...you compile in the Release configuration.

You need to set up C# 7.1 in Release, too. You could also remove the condition entirely, which sets up the language version for any configuration.