Missing netstandard 2.0 reference in Xamarin.Forms project when building with Jenkins

Question

I'm getting over 1500 errors like this, when I try to build an updated Xamarin.Forms project with Jenkins:

App.xaml.cs(24,32): error CS0012: Der Typ "Object" ist in einer nicht referenzierten Assembly definiert. Fügen Sie einen Verweis auf die Assembly "netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51" hinzu.

Translated error:

CS0012: The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

If I build the project with the same machine, but within Visual Studio 2017 it works without problems (as on my local machine). The build command for Jenkins looks like this

"C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\MSBuild" /p:ReferencePath=C:\jenkins\workspace\somelibrary\ SomeProject/SomeProject/SomeProject.csproj /t:Restore /p:Configuration=Release /t:Build 

Previously I updated to .NET Standard 2.0 and Xamarin.Forms 3.4.0.1039999. The build server itself is running on Windows 10 1703Windows 10 1909. I have UWP in my Xamarin.Forms project, but I'm not really using it. (I read that you need at least Windows 10 1709 if you want to use .NET Standard 2.0 together with UWP.)

The project is also referencing a NuGet package, which isn't compatible with .NET Standard 2.0 yet. Here I get the warning

Warning NU1701 Package 'xxx' was restored using '.NETFramework,Version=v4.6.1' instead of the project target framework '.NETStandard,Version=v2.0'. This package may not be fully compatible with your project.

This could be a reason why there are some problems, because it is not fully compatible.

What I tried:

• clean build, deleted bin and obj folder
• use of <Reference Include="netstandard" /> without success at building with Jenkins. Reference to NETStandard.Library throws a warning in my local build environment (but it is installed)
• updated Visual Studio Build Tools 2017 and Visual Studio 2017 to the latest version (15.9.18)
• installed .NET Core SDK, .NET Core Build Tools, .NET Framework 4.7.2 SDK on the build server
• extended PATH environment variable with C:\Program Files\dotnet, set global environment variable MSBuildSDKsPath to C:\Program Files\dotnet\sdk\2.1.509\Sdks
• deleted UWP project (still exists on folder, but not in *.sln)
• made some research after similar problems from other users, but didn't find something that solved the issue

What I observed:

• building the project works fine with Visual Studio 2017 on the build server with Windows 10 1703
• building the project works fine with Visual Studio 2017 on my local machine with Windows 10 1809
• building the project works fine with msbuild and command line on the build server (NuGet packages references correctly netstandard2.0 instead of netstandard1.3 and so on in the NuGetFallbackFolder)

What I could try:

• update the build server to Windows 10 1709 or higher
• replace/update the library, which still uses .NET 4.6.1
• upgrade to Visual Studio 2019
• create a .NET Standard project from scratch and copy all files over (and loosing GIT history)

But what I don't get is that it is working fine in Visual Studio but not with msbuild/Jenkins. Seems that a reference is missing or something is not installed.

What should I do?

Edit:

Extract of the .csproj from the Xamarin.Forms part of the project

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <NeutralLanguage>en</NeutralLanguage>
    <AssemblySearchPaths>
        $(AssemblySearchPaths);
        $(ReferencePath);
    </AssemblySearchPaths>
  </PropertyGroup>
  <ItemGroup>
    <None Remove="picture.png" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="Plugin" Version="5.2.0" />
  </ItemGroup>

  <ItemGroup>
    <Reference Include="SomeLibrary">
      <HintPath>pat\to\library\netstandard2.0\SomeLibrary.dll</HintPath>
    </Reference>
  </ItemGroup>

    <ItemGroup>
        <Compile Update="Common\Localization\AppResources.Designer.cs">
            <AutoGen>True</AutoGen>
            <DesignTime>True</DesignTime>
            <DependentUpon>AppResources.resx</DependentUpon>
        </Compile>
    </ItemGroup>
    <ItemGroup>
        <EmbeddedResource Update="Common\Localization\AppResources.de.resx">
            <SubType>Designer</SubType>
        </EmbeddedResource>
        <EmbeddedResource Update="Common\Localization\AppResources.resx">
            <Generator>PublicResXFileCodeGenerator</Generator>
            <LastGenOutput>AppResources.Designer.cs</LastGenOutput>
            <SubType>Designer</SubType>
        </EmbeddedResource>
    </ItemGroup>
    <ItemGroup>
      <None Update="App.xaml">
        <Generator>MSBuild:Compile</Generator>
      </None>
      <None Update="SomePage.xaml">
        <Generator>MSBuild:Compile</Generator>
      </None>
    </ItemGroup>

</Project>

Answer 1

I think I solved it with this

"C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\MSBuild" SomeProject/SomeProject/SomeProject.csproj /t:Restore /p:Configuration=Release

"C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\Bin\MSBuild" /p:ReferencePath=C:\jenkins\workspace\somelibrary\ SomeProject/SomeProject/SomeProject.csproj /p:Configuration=Release /t:Build

So the solution is to make two calls: one for restore and one for build. I do this for the Xamarin.Forms project and platform specific project separately.