Add <LangVersion>6</LangVersion> to the core protobuf C# project to prevent future incompatibility
diff --git a/csharp/src/Google.Protobuf/Google.Protobuf.csproj b/csharp/src/Google.Protobuf/Google.Protobuf.csproj
index 8ea3818..e65d396 100644
--- a/csharp/src/Google.Protobuf/Google.Protobuf.csproj
+++ b/csharp/src/Google.Protobuf/Google.Protobuf.csproj
@@ -5,6 +5,7 @@
<Copyright>Copyright 2015, Google Inc.</Copyright>
<AssemblyTitle>Google Protocol Buffers</AssemblyTitle>
<VersionPrefix>3.6.1</VersionPrefix>
+ <LangVersion>6</LangVersion>
<Authors>Google Inc.</Authors>
<TargetFrameworks>netstandard1.0;net45</TargetFrameworks>
<GenerateDocumentationFile>true</GenerateDocumentationFile>