Thanks for the answer. Is there any plan to add HAS_UNO to the Uno.UI package? Or can I use a non-empty UnoUIUseRoslynSourceGenerators value as an Uno platform indicator?

HAS_UNO is part of the Uno.UI package as well. It's used by the dotnet templates.

I create a DefineConstants property for the generator like this (taken from Uno Source Generator):

<Target Name="CreateDefineConstants" BeforeTargets="GenerateMSBuildEditorConfigFileShouldRun;GenerateMSBuildEditorConfigFileCore">

    <PropertyGroup>
      <DependencyPropertyGenerator_DefineConstants>$(DefineConstants.Replace(';',','))</DependencyPropertyGenerator_DefineConstants>
    </PropertyGroup>

  </Target>

As I understand it, the generator incorrectly perceives properties containing ";", so using the original DefineConstants is not possible.

But this method does not see the constants declared by the Uno package (as I understand it, this happens right before the code is compiled, after the generators have been started):

Can I provide a PR that moves the declaration of Uno constants to an earlier stage?

Sure!