diff --git a/csharp/src/Google.Protobuf.Test.CodeGen/ForeignMessagePartial.cs b/csharp/src/Google.Protobuf.Test.CodeGen/ForeignMessagePartial.cs
index 5663a69902..909bcea96e 100644
--- a/csharp/src/Google.Protobuf.Test.CodeGen/ForeignMessagePartial.cs
+++ b/csharp/src/Google.Protobuf.Test.CodeGen/ForeignMessagePartial.cs
@@ -39,7 +39,7 @@ namespace Google.Protobuf.TestProtos
     {
         public string ToDiagnosticString()
         {
-            return $"{{ \"c\": {C}, \"@cInHex\": \"{C:x}\" }}";
+            return string.Format("{{ \"c\": {0}, \"@cInHex\": \"{0:x}\" }}", C);
         }
     }
 }
diff --git a/csharp/src/Google.Protobuf.Test.CodeGen/Google.Protobuf.Test.CodeGen.csproj b/csharp/src/Google.Protobuf.Test.CodeGen/Google.Protobuf.Test.CodeGen.csproj
index dd4cd84e56..d756b0f757 100644
--- a/csharp/src/Google.Protobuf.Test.CodeGen/Google.Protobuf.Test.CodeGen.csproj
+++ b/csharp/src/Google.Protobuf.Test.CodeGen/Google.Protobuf.Test.CodeGen.csproj
@@ -1,7 +1,13 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
+  <!--
+    This CodeGen project is kept seperate from the original test project for many reasons.
+    It allows us to make sure code can compile on a seperate compiler version, different frameworks,
+    and without the internal visibility from the test project (all of which have caused issues in the past).
+  -->
   <PropertyGroup>
-    <TargetFrameworks>net451;netstandard1.0</TargetFrameworks>
+    <TargetFrameworks>net451;netstandard1.0;netstandard2.0</TargetFrameworks>
+    <LangVersion>3.0</LangVersion>
     <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
     <SignAssembly>true</SignAssembly>
     <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>