'AddImplicitDefineConstants' runs too late, and defines are missing when 'XamlPreCompile' runs, causing build failures

[Sergio0694]

Describe the bug

WinUI 3 and UWP apps (perhaps WPF/MAUI ones too?) use the XamlPreCompile task defined in the .NET SDK, which creates the intermediate .dll necessary for XAML compilation. The AddImplicitDefineConstants target runs before 'CoreCompile', but this seems to be too late and after 'XamlPreCompile' has already run. This causes the intermediate XAML build to not see the _OR_GREATER defines, meaning it will fail when those are used to exclude incompatible code, and resulting in very weird errors for someone looking at the IDE (which will show the code as being completely fine).

I hit this while migrating the Microsoft Store to .NET 9.

Can we make this target use BeforeTargets="BeforeCompile" or something, to address this?

To Reproduce

Create a WinUI 3 or UWP .NET 9 app with the following code behind:

#if !NET8_0_OR_GREATER
#error GenerateNETCompatibleDefineConstants didn't run 
#endif

Expected result

Should build as expected and match the behavior in the IDE.

Actual result

Further technical details

• Binlog (MSFT only)
• VS 17.12 P3