T4 TransformOnBuild, how to ensure dependent assemblies are copied into $(TargetDir) - msbuild

Question

I am trying to create a NuGet package that deploys some T4 .ttinclude files which in turn rely on a number of referenced assemblies. But I'm having issues in consuming projects when I try to enable
The .ttinclude files have been designed to reference required assemblies using the following syntax:
<## assembly name="$(TargetDir)WidgetDatabase.dll" #>
In my NuGet package's target file, I configure T4ParameterValues so that the $(TargetDir) is passed to the T4 text transformer:
<ItemGroup>
<T4ParameterValues Include="TargetDir">
<Value>$(TargetDir)</Value>
<Visible>false</Visible>
</T4ParameterValues>
</ItemGroup>
In the target .csproj, I have the following:
<PropertyGroup>
<TransformOnBuild>true</TransformOnBuild>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="WidgetDatabase">
<Version>0.8.1-alpha</Version>
</PackageReference>
</ItemGroup>
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
<Import Project="$(MSBuildExtensionsPath)\Microsoft\VisualStudio\v16.0\TextTemplating\Microsoft.TextTemplating.targets" />
The problem is that the T4 Text Transform occurs before the build, but this means that none of the referenced assemblies are copied into $(TargetDir). How can I force the dependencies to be copied into the $(TargetDir) prior to the text transform target running?
This question is related, but doesn't satisfactorily resolve this issue.

Answer 1

I was facing the same problem and I managed to solve this by referencing the assembly from the NuGet package folder instead of trying to copy it to $(TargetDir).
NuGet 5.0 provides a GeneratePathProperty property that you can add to the PackageReference. This will create a MSBuild property that points to the location from which the package will be consumed:
<ItemGroup>
<PackageReference Include="WidgetDatabase" GeneratePathProperty="true">
<Version>0.8.1-alpha</Version>
</PackageReference>
</ItemGroup>
This property can then be added to T4ParameterValues so it can be consumed from your T4 template:
<ItemGroup>
<T4ParameterValues Include="PkgWidgetDatabase">
<Value>$(PkgWidgetDatabase)</Value>
<Visible>false</Visible>
</T4ParameterValues>
</ItemGroup>
<## assembly name="$(PkgWidgetDatabase)\lib\netstandard2.0\WidgetDatabase.dll" #>
The nice thing about this approach is that it works in MSBuild and in Visual Studio design-time.