Predefined type 'System.Void' is not defined or imported during Visual Studio Online build of ASP.NET 5

I get this error when I try to build the project in Visual Studio Online:

…\Frontend\Startup.cs(65,49): Error CS0518: Predefined type 'System.Void' is not defined or imported
…\Frontend\Startup.cs(65,49): DNXCore,Version=v5.0 error CS0518: Predefined type 'System.Void' is not defined or imported [C:\a\1\s\Frontend\src\Frontend\Frontend.xproj]

It seems that the error happens in the place where I reference another library (package class library).

P.S. I had some conversation about this here: Easy way to build and deploy (to Azure) ASP.NET 5 in Visual Studio Team Services, but there was no answer after all, so I still cannot build it...

Web project.json:

"dependencies": {
    "Microsoft.AspNet.IISPlatformHandler": "1.0.0-rc1-final",
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final",
    "Microsoft.AspNet.Mvc.TagHelpers": "6.0.0-rc1-final",
    "Microsoft.AspNet.Server.Kestrel": "1.0.0-rc1-final",
    "Microsoft.AspNet.StaticFiles": "1.0.0-rc1-final",
    "Microsoft.AspNet.Tooling.Razor": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.FileProviderExtensions": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.Json": "1.0.0-rc1-final",
    "Microsoft.Extensions.Configuration.UserSecrets": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging.Console": "1.0.0-rc1-final",
    "Microsoft.Extensions.Logging.Debug": "1.0.0-rc1-final",
    "Web.Common": "1.0.0-*"
},

"frameworks": {
    "dnx451": { },
    "dnxcore50": { }
},

Library project.json

"frameworks": {
    "dnx451": { },
    "dnxcore50": {
        "dependencies": {
            "Microsoft.CSharp": "4.0.1-beta-23516",
            "System.Collections": "4.0.11-beta-23516",
            "System.Linq": "4.0.1-beta-23516",
            "System.Runtime": "4.0.21-beta-23516",
            "System.Threading": "4.0.11-beta-23516"
      }
    }
  },
"dependencies": {
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final"
  }

Update (problem and solution):

The problem was in the fact that my library reference was not in the same folder and the script by default does not run dnu-restore there, so I had to go one level up and start finding project.json recursively there

$SourceRoot = Split-Path -Path $PSScriptRoot -Parent

# run DNU restore on all project.json files in the src folder including 2>1 to redirect stderr to stdout for badly behaved tools
Get-ChildItem -Path $SourceRoot -Filter project.json -Recurse | ForEach-Object { & dnu restore $_.FullName 2>1 }

Thanks Eddie - MSFT for help!

Solution 1:

In my own issue, I rebuilt the entire solution and the errors just disappeared.

Solution 2:

I just reproduced this issue after updating the code in "PreBuild.ps1" script to only restore the "project.json" file for web project.

Original Code (Build Success):

Get-ChildItem -Path $PSScriptRoot\src -Filter project.json -Recurse | ForEach-Object { & dnu restore $_.FullName 2>1 }

Updated Code (Build fail with: Predefined type 'System.Void' is not defined or imported):

Get-ChildItem -Path $PSScriptRoot\src\WebProjectName -Filter project.json -Recurse | ForEach-Object { & dnu restore $_.FullName 2>1 }

So you need to check the PreBuild.ps1 script and the build logs for PowerShell step to check if the project.json files for Web project and Library project are both restored.

Solution 3:

In my experience, I deleted the bin and obj folder and it worked. However, for my colleague he deleted the entire solution folder and got the latest from TFS. He then compiled and rebuilt the solution and it worked.