A source code nuget package defining a .cs.pp file fails to resolve the namespace if <RootNamespace> is not set in the project

[vpenades]

Describe the bug

Originally posted here: NuGet/Home#13346

.cs.pp template files coming from a source code nuget package fail to evaluate the $rootnamespace$ macro in the .cs.pp files, because <RootNamespace> definition is missing in the csproj file.

To Reproduce

Create a source code package including a .cs.pp template file that uses namespace $rootnamespace$ ... or consume an existing source code nuget like CodeSugar.Sys.IO.Sources into a new project.

The test project referencing the nuget package does NOT have a <RootNamespace> property defined, so theoretically, the default assembly namespace should be used for $rootnamespace$

And what happens is this:

-At edition time, the files seem to be correctly evaluated with the correct namespace, and the files and classes are correctly found by intellisense within visual studio.
-At build time, it fails and gives errors saying that the templated files are missing due to (I guess) incorrectly resolving $rootnamespace$

If then, I add the <RootNamespace> to the test project, even if the namespace is the same as the default, then the project builds correctly.

So clearly, there's a problem at build time, the compiler is unable to resolve the root namespace if it's not explicitly defined using the <RootNamespace> property.

in code:

<project>

<PropertyGroup>
  <!-- <RootNamespace>DefaultAssemblyNamespace</RootNamespace> -->   <== if I comment this line, it fails to build
</PropertyGroup>

<ItemGroup>
  <PackageReference Include="CodeSugar.Sys.IO.Sources" Version="1.0.0-Preview-20240326-110152" PrivateAssets="all" />
</ItemGroup>

</project>

Exceptions (if any)

Further technical details

• Visual Studio Version 17.9.5