Directory.Build.props: Added section to compare Visual Studio versions (including prerelease versions) to use to drive IsLegacyVisualStudioVersion property across the solution

Author: NightOwl888

Directory.Build.props

@@ -31,17 +31,24 @@
     <GitHubProject>lucenenet</GitHubProject>
   </PropertyGroup>
 
+  <PropertyGroup Label="Legacy Visual Studio Support">
+    <VisualStudioMajorVersion>0</VisualStudioMajorVersion>
+    <!-- Extract major version (works for preview builds too) -->
+    <VisualStudioMajorVersion Condition="'$(VisualStudioVersion)' != ''">$([System.Version]::Parse($([System.Text.RegularExpressions.Regex]::Match('$(VisualStudioVersion)', '^\d+(\.\d+)*').Value)).Major)</VisualStudioMajorVersion>
+    <IsLegacyVisualStudioVersion>false</IsLegacyVisualStudioVersion>
+    <!-- Visual Studio 2026 == Version 18.x - anything below that major version is legacy -->
+    <IsLegacyVisualStudioVersion Condition="'$(BuildingInsideVisualStudio)' == 'true' And '$(VisualStudioMajorVersion)' &lt; '18'">true</IsLegacyVisualStudioVersion>
+  </PropertyGroup>
+
   <PropertyGroup Label="Centralized Target Frameworks">
-    <!-- Base frameworks without net10.0 -->
-    <LibraryTargetFrameworksBase>net8.0;netstandard2.0;net462</LibraryTargetFrameworksBase>
-    <NetCoreOnlyTargetFrameworksBase>net8.0</NetCoreOnlyTargetFrameworksBase>
+    <!-- net10.0 is conditional on VS version (VS2024+ = 18.0) -->
+    <!-- .NET Framework and netstandard2.0 are conditional on VS version (VS2024+ = 18.0) due to invalid build warnings with C# 14.0 build features -->
 
-    <!-- Add net10.0 conditionally based on VS version (VS2024+ = 18.0) -->
-    <LibraryTargetFrameworks Condition=" '$(VisualStudioVersion)' &gt;= '18.0' ">net10.0;$(LibraryTargetFrameworksBase)</LibraryTargetFrameworks>
-    <LibraryTargetFrameworks Condition=" '$(VisualStudioVersion)' &lt; '18.0' Or '$(VisualStudioVersion)' == '' ">$(LibraryTargetFrameworksBase)</LibraryTargetFrameworks>
+    <LibraryTargetFrameworks>net10.0;net8.0;netstandard2.0;net462</LibraryTargetFrameworks>
+    <LibraryTargetFrameworks Condition=" '$(IsLegacyVisualStudioVersion)' == 'true' ">net8.0</LibraryTargetFrameworks>
 
-    <NetCoreOnlyTargetFrameworks Condition=" '$(VisualStudioVersion)' &gt;= '18.0' ">net10.0;$(NetCoreOnlyTargetFrameworksBase)</NetCoreOnlyTargetFrameworks>
-    <NetCoreOnlyTargetFrameworks Condition=" '$(VisualStudioVersion)' &lt; '18.0' Or '$(VisualStudioVersion)' == '' ">$(NetCoreOnlyTargetFrameworksBase)</NetCoreOnlyTargetFrameworks>
+    <NetCoreOnlyTargetFrameworks>net10.0;net8.0</NetCoreOnlyTargetFrameworks>
+    <NetCoreOnlyTargetFrameworks Condition=" '$(IsLegacyVisualStudioVersion)' == 'true' ">net8.0</NetCoreOnlyTargetFrameworks>
   </PropertyGroup>
 
   <!-- IMPORTANT: When these values are changed, the CI counter number should also be reset. -->