roslyn: Unable to compile C# 7.1 code on VSTS build agents

Visual Studio Enterprise 2017 v15.3.2 .NET framework 4.7.02046 VSTS Build Agent: 2.120.2 (running on Server 2008, latest patches)

Steps to Reproduce:

1. Create c# console app for .NET 4.7
2. Change void Main() to async Task Main()
3. Change project to use C# 7.1
4. Compile and run on dev machine
5. Push repo to VSTS system
6. Create a build: use “.NET Desktop” template
7. Use local build queue
8. Check “build solution” is set to “latest” Visual Studio version
9. Save & Queue build

CSharp7_1_Test.zip

Expected Behavior:

Build completes and recognises async Task Main as valid C# 7.1

Actual Behavior:

Build fails with error:

2017-08-29T08:49:18.0026052Z ##[error]Program.cs(11,22): Error CS8107: Feature 'async main' is not available in C# 7. Please use language version 7.1 or greater.

Adding Roslyn compilers package does not fix this. Same error.

Notes:

First attempt on build server as 4.7 failed as the SDK was not installed (odd, since I’d installed VS 2017 on the machine earlier). Downloaded and ran NDP47-DevPack-KB3186612-ENU.exe on the build server to fix this.

Second build failed as expected (log below):

2017-08-29T08:48:44.2515948Z CoreCompile:
2017-08-29T08:48:44.2672205Z   E:\Microsoft Visual Studio 2017\Enterprise\MSBuild\15.0\Bin\Roslyn\csc.exe /noconfig /nowarn:1701,1702 /nostdlib+ /platform:anycpu32bitpreferred /errorreport:prompt /warn:4 /define:TRACE /highentropyva+ /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\Microsoft.CSharp.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\mscorlib.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Core.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Data.DataSetExtensions.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Data.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Net.Http.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Xml.dll" /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.7\System.Xml.Linq.dll" /debug:pdbonly /filealign:512 /optimize+ /out:obj\Release\CSharp7_1_Test.exe /ruleset:"E:\Microsoft Visual Studio 2017\Enterprise\Team Tools\Static Analysis Tools\\Rule Sets\MinimumRecommendedRules.ruleset" /subsystemversion:6.00 /target:exe /utf8output Program.cs Properties\AssemblyInfo.cs "D:\BuildAgent1\_work\_temp\.NETFramework,Version=v4.7.AssemblyAttributes.cs"
2017-08-29T08:48:45.2516396Z   Using shared compilation with compiler from directory: E:\Microsoft Visual Studio 2017\Enterprise\MSBuild\15.0\Bin\Roslyn
2017-08-29T08:49:18.0026052Z ##[error]Program.cs(11,22): Error CS8107: Feature 'async main' is not available in C# 7. Please use language version 7.1 or greater.
2017-08-29T08:49:18.0026052Z Program.cs(11,22): error CS8107: Feature 'async main' is not available in C# 7. Please use language version 7.1 or greater. [D:\BuildAgent1\_work\89\s\CSharp7_1_Test.csproj]
2017-08-29T08:49:18.0807322Z ##[error]CSC(0,0): Error CS5001: Program does not contain a static 'Main' method suitable for an entry point
2017-08-29T08:49:18.0807322Z CSC : error CS5001: Program does not contain a static 'Main' method suitable for an entry point [D:\BuildAgent1\_work\89\s\CSharp7_1_Test.csproj]
2017-08-29T08:49:18.1588592Z Done Building Project "D:\BuildAgent1\_work\89\s\CSharp7_1_Test.csproj" (default targets) -- FAILED.

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[edit] I also tested by using the “Hosted VS2017” build agent queue - this failed with the same error.

Thanks to @jcouv for taking a look at this.