Try this

Author: mnadareski

SabreTools.Serialization/SabreTools.Serialization.csproj

@@ -26,6 +26,14 @@
         <PackageLicenseExpression>MIT</PackageLicenseExpression>
     </PropertyGroup>
 
+    <!-- Set a build flag for Windows specifically -->
+    <PropertyGroup Condition="'$(RuntimeIdentifier)'=='win-x86'">
+        <DefineConstants>$(DefineConstants);WINX86</DefineConstants>
+    </PropertyGroup>
+    <PropertyGroup Condition="'$(RuntimeIdentifier)'=='win-x64'">
+        <DefineConstants>$(DefineConstants);WINX64</DefineConstants>
+    </PropertyGroup>
+
     <!-- Exclude certain parts of external modules for by default -->
     <PropertyGroup>
         <DefaultItemExcludes>