In your project file :
<Compile Include="FileA.cs"/>
<Compile Include="FileA.xml">
<DependentUpon>FileA.cs</DependentUpon>
</Compile>
Or you could use Group Items
command of VSCommands 2010 extension.
Edit: Just in case your file is in a folder, don’t include the folder name in DependentUpon
tag. For example if your file is in Helpers
folder:
<Compile Include="Helpers\FileA.cs"/>
<Compile Include="Helpers\FileA.xml">
<DependentUpon>FileA.cs</DependentUpon>
</Compile>