Finds .cs files that aren't included in a .csproj file.

VerifyAllSourceFilesAreIncluded.proj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="3.5" DefaultTargets="Verify" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="SomeProject.csproj" />

  <ItemGroup>
    <ShouldBeIncluded Include="**/*.cs" Exclude="@(Compile)" />
  </ItemGroup>

  <Target Name="Verify">
    <Error Text="The following cs files are on disk, but not in the csproj: @(ShouldBeIncluded)" Condition="'@(ShouldBeIncluded)' != ''" />
  </Target>
</Project>