diff --git a/lib/csharp/src/Thrift.csproj b/lib/csharp/src/Thrift.csproj
index e3022a4..83bc4f7 100644
--- a/lib/csharp/src/Thrift.csproj
+++ b/lib/csharp/src/Thrift.csproj
@@ -79,6 +79,7 @@
   <ItemGroup>
     <Compile Include="Collections\TCollections.cs" />
     <Compile Include="Collections\THashSet.cs" />
+    <Compile Include="Net35\ExtensionsNet35.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Protocol\TAbstractBase.cs" />
     <Compile Include="Protocol\TBase.cs" />