diff --git a/Makefile.am b/Makefile.am
index 755355f421c7..8fd66c79ee2b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -127,21 +127,22 @@ csharp_EXTRA_DIST=                                                           \
   csharp/src/Google.Protobuf.Test/SampleMessages.cs                          \
   csharp/src/Google.Protobuf.Test/SampleNaNs.cs                              \
   csharp/src/Google.Protobuf.Test/TestCornerCases.cs                         \
-  csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs        \
-  csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs            \
-  csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions1.cs               \
-  csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions2.cs               \
-  csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto2.cs           \
-  csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs           \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs  \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs         \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs   \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublic.cs         \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestImport.cs               \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs               \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs               \
-  csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs       \
-  csharp/src/Google.Protobuf.Test/TestProtos/Unittest.cs                     \
+  csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj \
+  csharp/src/Google.Protobuf.Test.TestProtos/ForeignMessagePartial.cs           \
+  csharp/src/Google.Protobuf.Test.TestProtos/MapUnittestProto3.cs               \
+  csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions1.cs                  \
+  csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions2.cs                  \
+  csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto2.cs              \
+  csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto3.cs              \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestCustomOptionsProto3.cs     \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportProto3.cs            \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublicProto3.cs      \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublic.cs            \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestImport.cs                  \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestIssues.cs                  \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestProto3.cs                  \
+  csharp/src/Google.Protobuf.Test.TestProtos/UnittestWellKnownTypes.cs          \
+  csharp/src/Google.Protobuf.Test.TestProtos/Unittest.cs                        \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/AnyTest.cs                  \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/DurationTest.cs             \
   csharp/src/Google.Protobuf.Test/WellKnownTypes/FieldMaskTest.cs            \
diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 1083d671c1d6..66d87c02f711 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -45,7 +45,7 @@ $PROTOC -Isrc --csharp_out=csharp/src/Google.Protobuf \
 # and old_extensions2.proto, which are generated with an older version
 # of protoc.
 $PROTOC -Isrc -Icsharp/protos \
-    --csharp_out=csharp/src/Google.Protobuf.Test/TestProtos \
+    --csharp_out=csharp/src/Google.Protobuf.Test.TestProtos \
     --descriptor_set_out=csharp/src/Google.Protobuf.Test/testprotos.pb \
     --include_source_info \
     --include_imports \
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs b/csharp/src/Google.Protobuf.Test.TestProtos/ForeignMessagePartial.cs
similarity index 96%
rename from csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/ForeignMessagePartial.cs
index 5663a69902e5..909bcea96e59 100644
--- a/csharp/src/Google.Protobuf.Test/TestProtos/ForeignMessagePartial.cs
+++ b/csharp/src/Google.Protobuf.Test.TestProtos/ForeignMessagePartial.cs
@@ -39,7 +39,7 @@ public partial class ForeignMessage : ICustomDiagnosticMessage
     {
         public string ToDiagnosticString()
         {
-            return $"{{ \"c\": {C}, \"@cInHex\": \"{C:x}\" }}";
+            return string.Format("{{ \"c\": {0}, \"@cInHex\": \"{0:x}\" }}", C);
         }
     }
 }
diff --git a/csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj b/csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj
new file mode 100644
index 000000000000..5b705804aa27
--- /dev/null
+++ b/csharp/src/Google.Protobuf.Test.TestProtos/Google.Protobuf.Test.TestProtos.csproj
@@ -0,0 +1,26 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    This TestProtos project is kept separate from the original test project for many reasons.
+    It allows us to make sure code can compile on a separate compiler version, different frameworks,
+    and without the internal visibility from the test project (all of which have caused issues in the past).
+  -->
+  <PropertyGroup>
+    <TargetFrameworks>net45;netstandard1.0;netstandard2.0</TargetFrameworks>
+    <LangVersion>3.0</LangVersion>
+    <AssemblyOriginatorKeyFile>../../keys/Google.Protobuf.snk</AssemblyOriginatorKeyFile>
+    <SignAssembly>true</SignAssembly>
+    <PublicSign Condition=" '$(OS)' != 'Windows_NT' ">true</PublicSign>
+    <IsPackable>False</IsPackable>
+  </PropertyGroup>
+
+  <!-- Needed for the net45 build to work on Unix. See https://github.com/dotnet/designs/pull/33 -->
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
+  </ItemGroup>
+  
+  <ItemGroup>
+    <ProjectReference Include="..\Google.Protobuf\Google.Protobuf.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/MapUnittestProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/MapUnittestProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/MapUnittestProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions1.cs b/csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions1.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions1.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions1.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions2.cs b/csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions2.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/OldExtensions2.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/OldExtensions2.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto2.cs b/csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto2.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto2.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto2.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/TestMessagesProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/TestMessagesProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/Unittest.cs b/csharp/src/Google.Protobuf.Test.TestProtos/Unittest.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/Unittest.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/Unittest.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestCustomOptionsProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestCustomOptionsProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestCustomOptionsProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImport.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestImport.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestImport.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestImport.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublic.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublic.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublic.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublic.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublicProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestImportPublicProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestImportPublicProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestIssues.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestIssues.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestIssues.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestProto3.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestProto3.cs
diff --git a/csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs b/csharp/src/Google.Protobuf.Test.TestProtos/UnittestWellKnownTypes.cs
similarity index 100%
rename from csharp/src/Google.Protobuf.Test/TestProtos/UnittestWellKnownTypes.cs
rename to csharp/src/Google.Protobuf.Test.TestProtos/UnittestWellKnownTypes.cs
diff --git a/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj b/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
index 7fff91283b90..1a7953d6b786 100644
--- a/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
+++ b/csharp/src/Google.Protobuf.Test/Google.Protobuf.Test.csproj
@@ -10,6 +10,7 @@
 
   <ItemGroup>
     <ProjectReference Include="..\Google.Protobuf\Google.Protobuf.csproj" />
+    <ProjectReference Include="..\Google.Protobuf.Test.TestProtos\Google.Protobuf.Test.TestProtos.csproj"/>
   </ItemGroup>
 
   <ItemGroup>
diff --git a/csharp/src/Google.Protobuf.sln b/csharp/src/Google.Protobuf.sln
index 262a43b78f3b..b571e53c12eb 100644
--- a/csharp/src/Google.Protobuf.sln
+++ b/csharp/src/Google.Protobuf.sln
@@ -1,4 +1,4 @@
-Microsoft Visual Studio Solution File, Format Version 12.00
+Microsoft Visual Studio Solution File, Format Version 12.00
 # Visual Studio 15
 VisualStudioVersion = 15.0.26114.2
 MinimumVisualStudioVersion = 10.0.40219.1
@@ -14,6 +14,8 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Google.Protobuf.JsonDump",
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Google.Protobuf.Benchmarks", "Google.Protobuf.Benchmarks\Google.Protobuf.Benchmarks.csproj", "{D25E4804-4DEA-45AB-9F8C-BA4DBD8E5A07}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Google.Protobuf.Test.TestProtos", "Google.Protobuf.Test.TestProtos\Google.Protobuf.Test.TestProtos.csproj", "{ADF24BEB-A318-4530-8448-356B72B820EA}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -44,6 +46,10 @@ Global
 		{D25E4804-4DEA-45AB-9F8C-BA4DBD8E5A07}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{D25E4804-4DEA-45AB-9F8C-BA4DBD8E5A07}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{D25E4804-4DEA-45AB-9F8C-BA4DBD8E5A07}.Release|Any CPU.Build.0 = Release|Any CPU
+		{ADF24BEB-A318-4530-8448-356B72B820EA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{ADF24BEB-A318-4530-8448-356B72B820EA}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{ADF24BEB-A318-4530-8448-356B72B820EA}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{ADF24BEB-A318-4530-8448-356B72B820EA}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE