Run 'AddImplicitDefineConstants' on 'BeforeCompile'

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.BeforeCommon.targets

@@ -251,11 +251,18 @@ Copyright (c) .NET Foundation. All rights reserved.
     </PropertyGroup>
   </Target>
 
-  <!-- Add implicitly defined preprocessor symbols to DefineConstants -->
+  <!--
+    Add implicitly defined preprocessor symbols to DefineConstants. Note: this is intentionally using 'BeforeCompile' as
+    trigger to ensure that it runs before the XamlPreCompile targets. This targets creates the intermediate XAML project
+    that is used by some XAML-based UI frameworks (eg. UWP XAML, WinUI 3). If it runs before AddImplicitDefineConstants,
+    and the code has any '#ifdefs' on constants generated by this target (eg. 'NET9_0_OR_GREATER'), the intermediate XAML
+    project will end up selecting the other code paths instead, which can cause all kinds of build failures or other
+    problems. For additional context, see: https://github.com/dotnet/sdk/issues/43908.
+  -->
   <Target Name="AddImplicitDefineConstants"
           Condition=" '$(DisableImplicitFrameworkDefines)' != 'true' "
           DependsOnTargets="GenerateTargetPlatformDefineConstants;GenerateNETCompatibleDefineConstants;GeneratePlatformCompatibleDefineConstants;_DisableDiagnosticTracing"
-          BeforeTargets="CoreCompile" >
+          BeforeTargets="BeforeCompile" >
     <PropertyGroup>
       <DefineConstants Condition=" '@(_ImplicitDefineConstant)' != '' " >$(DefineConstants);@(_ImplicitDefineConstant)</DefineConstants>
       <FinalDefineConstants Condition=" '@(_ImplicitDefineConstant)' != '' and '$(Language)' == 'VB' " >$(FinalDefineConstants),@(_ImplicitDefineConstant->'%(Identity)=-1', ',')</FinalDefineConstants>