Upgraded C# LangVersion to 8.0
diff --git a/Directory.Build.props b/Directory.Build.props
index 4cb9e84..20c96ce 100644
--- a/Directory.Build.props
+++ b/Directory.Build.props
@@ -21,6 +21,10 @@
 <Project>
   <Import Project="build\Dependencies.props" />
 
+  <PropertyGroup>
+    <LangVersion>8.0</LangVersion>
+  </PropertyGroup>
+  
   <!-- IMPORTANT: When these values are changed, the CI counter number should also be reset. -->
   <PropertyGroup Label="Version of Builds">
     <!-- IMPORTANT: VersionPrefix must always be the same as the Lucene version this is based on.