build fails when targeting netcoreapp3.1 and net5.0

[stefanloerwald]

This is an ugly one for people upgrading from netcoreapp3.1 to net5.0..

The setup:

• An empty (no .cs files) class library project. Full csproj at path ClassLibrary1\ClassLibrary1.csproj:

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

  <PropertyGroup>
    <TargetFrameworks>netcoreapp3.1;net5.0</TargetFrameworks>
  </PropertyGroup>

</Project>

• A minimalistic test project (same: no .cs files). Full csproj at path NUnitTestProject1/NUnitTestProject1.csproj:

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

  <PropertyGroup>
    <TargetFrameworks>net5.0</TargetFrameworks>
    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <ProjectReference Include="..\ClassLibrary1\ClassLibrary1.csproj" />
  </ItemGroup>

</Project>

(Side note: the path suggests NUnit, but the reference was removed, as that's not the root of the issue at all.)

Now the build process:

FROM mcr.microsoft.com/dotnet/core/sdk:5.0-buster
COPY . .
RUN dotnet test NUnitTestProject1/NUnitTestProject1.csproj
RUN dotnet build -c Release -o /app/build

The build will fail on the last command. The error log is:

2020-09-03T08:00:50.5307809Z Step 4/4 : RUN dotnet build -c Release -o /app/build
2020-09-03T08:00:50.5496428Z  ---> Running in afaf6729f4fa
2020-09-03T08:00:51.2819871Z Microsoft (R) Build Engine version 16.7.0-preview-20360-03+188921e2f for .NET
2020-09-03T08:00:51.2820120Z Copyright (C) Microsoft Corporation. All rights reserved.
2020-09-03T08:00:51.2820238Z 
2020-09-03T08:00:53.0044398Z   Determining projects to restore...
2020-09-03T08:00:53.5041025Z   All projects are up-to-date for restore.
2020-09-03T08:00:53.6243808Z   You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
2020-09-03T08:00:53.6244559Z   You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
2020-09-03T08:00:54.8928782Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018: The "GenerateDepsFile" task failed unexpectedly. [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8929578Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018: System.IO.IOException: The process cannot access the file '/app/build/ClassLibrary1.deps.json' because it is being used by another process. [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8935929Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream.Init(FileMode mode, FileShare share, String originalPath) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8936631Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8943017Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8943612Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.File.Create(String path) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8944220Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.GenerateDepsFile.WriteDepsFile(String depsFilePath) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8948105Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.GenerateDepsFile.ExecuteCore() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8948685Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.TaskBase.Execute() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8952595Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8956187Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:54.8989765Z   ClassLibrary1 -> /app/build/ClassLibrary1.dll
2020-09-03T08:00:55.0730864Z   NUnitTestProject1 -> /app/build/NUnitTestProject1.dll
2020-09-03T08:00:55.0949798Z 
2020-09-03T08:00:55.0950154Z Build FAILED.
2020-09-03T08:00:55.0955028Z 
2020-09-03T08:00:55.0960284Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018: The "GenerateDepsFile" task failed unexpectedly. [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0978113Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018: System.IO.IOException: The process cannot access the file '/app/build/ClassLibrary1.deps.json' because it is being used by another process. [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0982645Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream.Init(FileMode mode, FileShare share, String originalPath) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0987064Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0992692Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0993265Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at System.IO.File.Create(String path) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0998724Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.GenerateDepsFile.WriteDepsFile(String depsFilePath) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.0999330Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.GenerateDepsFile.ExecuteCore() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.1006812Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.NET.Build.Tasks.TaskBase.Execute() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.1007449Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute() [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.1008224Z /usr/share/dotnet/sdk/5.0.100-preview.7.20366.15/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.Sdk.targets(195,5): error MSB4018:    at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(ITaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask) [/ClassLibrary1/ClassLibrary1.csproj]
2020-09-03T08:00:55.1008446Z     0 Warning(s)
2020-09-03T08:00:55.1008574Z     1 Error(s)
2020-09-03T08:00:55.1008644Z 
2020-09-03T08:00:55.1008769Z Time Elapsed 00:00:03.68
2020-09-03T08:00:55.2952741Z The command '/bin/sh -c dotnet build -c Release -o /app/build' returned a non-zero code: 1
2020-09-03T08:00:55.2960996Z ##[error]Process completed with exit code 1.

A solution (to those who can just break with the past...) is to remove the reference to netcoreapp3.1.

A full repo showing the issue is available here: https://github.com/stefanloerwald/dotnet-build-targetframeworks-bug. It includes a github action to demonstrate that the issue does not just exist on my machine.

[dsplaisted]

The problem is you're using the -o command line option on a multi-targeted project. So it's trying to build the output for both target frameworks to the same folder, which is causing errors.

I've filed #13369 for us to improve this.

[stefanloerwald]

So why does it work when not executing the test?

[Zastai]

Because then the parallel builds just overwrite the files silently. Running the tests means that the files are in use, so the overwrite can't happen.