Build project with multiple targetframeworks in TFS as a NuGet package - tfs

Question

I want to build my NuGet package in one of my TFS build steps.
Because .NET Standard 2.0 is currently not supported by the TFS NuGet step, I am using /t:pack as the build argument in my MSBuild invocation.
When I only use one TargetFramework it is working fine. My assemblies get a version and my NuGet package too.
Even if I am using an AssemblyInfo.cs in .NET Standard 2.0 - explained in this answer.
So far no problem, but if I want to build now on two different TargetFrameworks like <TargetFramework>netstandard2.0;net45</TargetFramework> my NuGet package won't get a Version.
So in the end this is working and is generating a versioned NuGet file.
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFramework>netstandard2.0</TargetFramework>
<GeneratePackageOnBuild>false</GeneratePackageOnBuild>
<GenerateNuspecDependsOn>$(GenerateNuspecDependsOn);ReadPackageVersionFromOutputAssembly</GenerateNuspecDependsOn>
<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
</PropertyGroup>
<Target Name="ReadPackageVersionFromOutputAssembly" DependsOnTargets="Build">
<GetAssemblyIdentity AssemblyFiles="$(TargetPath)">
<Output TaskParameter="Assemblies" ItemName="PackAssembly" />
</GetAssemblyIdentity>
<PropertyGroup>
<PackageVersion>%(PackAssembly.Version)</PackageVersion>
</PropertyGroup>
</Target>
<ItemGroup>
<Compile Remove="AccessControl\**" />
<EmbeddedResource Remove="AccessControl\**" />
<None Remove="AccessControl\**" />
</ItemGroup>
<ItemGroup>
<Compile Include="..\common\Version.cs" Link="Properties\Version.cs" />
</ItemGroup>
<ItemGroup>
<PackageReference Include="System.Threading.Tasks.Dataflow" Version="4.8.0" />
</ItemGroup>
<ItemGroup>
<Reference Include="MyEngine">
<HintPath>..\libs\NetStandard\MyEngine.dll</HintPath>
</Reference>
</ItemGroup>
<ItemGroup>
<Folder Include="Properties\" />
</ItemGroup>
</Project>
So here is my current .csproj file. This builds too and also generates a NuGet file. But in this case my NuGet file is unversioned. Why is it this way and how can I fix it?
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<TargetFrameworks>netstandard2.0;net45</TargetFrameworks>
<GeneratePackageOnBuild>false</GeneratePackageOnBuild>
<GenerateNuspecDependsOn>$(GenerateNuspecDependsOn);ReadPackageVersionFromOutputAssembly</GenerateNuspecDependsOn>
<GenerateAssemblyInfo>false</GenerateAssemblyInfo>
</PropertyGroup>
<Target Name="ReadPackageVersionFromOutputAssembly" DependsOnTargets="Build">
<GetAssemblyIdentity AssemblyFiles="$(TargetPath)">
<Output TaskParameter="Assemblies" ItemName="PackAssembly" />
</GetAssemblyIdentity>
<PropertyGroup>
<PackageVersion>%(PackAssembly.Version)</PackageVersion>
</PropertyGroup>
</Target>
<ItemGroup>
<Compile Remove="AccessControl\**" />
<EmbeddedResource Remove="AccessControl\**" />
<None Remove="AccessControl\**" />
</ItemGroup>
<ItemGroup>
<Compile Include="..\common\Version.cs" Link="Properties\Version.cs" />
</ItemGroup>
<ItemGroup>
<PackageReference Include="System.Threading.Tasks.Dataflow" Version="4.8.0" />
</ItemGroup>
<ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
<Reference Include="MyEngine">
<HintPath>..\libs\NetStandard\MyEngine.dll</HintPath>
</Reference>
</ItemGroup>
<ItemGroup Condition="'$(TargetFramework)' == 'net45'">
<Reference Include="MyEngine">
<HintPath>..\libs\NET45\MyEngine.dll</HintPath>
</Reference>
</ItemGroup>
<ItemGroup>
<Folder Include="Properties\" />
</ItemGroup>
</Project>

Answer 1

The targets can be adapted to support multi-targeting like this:
<Target Name="ReadPackageVersionFromOutputAssemblySingleTfm" Returns="#(PackAssembly)" Condition="'$(IsCrossTargetingBuild)' != 'true'">
<GetAssemblyIdentity AssemblyFiles="$(TargetPath)">
<Output TaskParameter="Assemblies" ItemName="PackAssembly" />
</GetAssemblyIdentity>
<PropertyGroup>
<PackageVersion>%(PackAssembly.Version)</PackageVersion>
</PropertyGroup>
</Target>
<Target Name="ReadPackageVersionFromOutputAssemblyMultipleTfms" Condition="'$(IsCrossTargetingBuild)' == 'true'">
<PropertyGroup>
<FirstTargetFramework>$([System.String]::Copy($(TargetFrameworks)).Split(';').GetValue(0))</FirstTargetFramework>
</PropertyGroup>
<MSBuild Projects="$(MSBuildProjectFullPath)" Targets="ReadPackageVersionFromOutputAssemblySingleTfm" Properties="TargetFramework=$(FirstTargetFramework)">
<Output TaskParameter="TargetOutputs" ItemName="PackAssembly" />
</MSBuild>
<PropertyGroup>
<PackageVersion>%(PackAssembly.Version)</PackageVersion>
</PropertyGroup>
</Target>
<Target Name="ReadPackageVersionFromOutputAssembly" DependsOnTargets="Build;ReadPackageVersionFromOutputAssemblySingleTfm;ReadPackageVersionFromOutputAssemblyMultipleTfms" />