message: [c#] Append to list in serialization example
  The previous code in the serialization example created a new
  List<double> for the vector<double> field `items`, implying that the
  field was not initialized to an empty List<double>.
  Since the `items` field is initialized to an empty list, we now just
  append to the existing list.
diff: </PropertyGroup>
  <ItemGroup>
  <Reference Include="System" />
  - <Reference Include="System.Core" />
  - <Reference Include="System.Xml" />
  - <Reference Include="Microsoft.CSharp" />
  - <Reference Include="System.Data" />
  </ItemGroup>
  <ItemGroup>
  <Compile Include="program.cs" />
  