Hi Douglas,

This is basicly a VS IDE issue instead of being controlled by .Net Winform
designer, so you'd better post in microsoft.public.vsnet.ide newsgroup in
future.

Anyway, this is actually an undocumented feature of VS IDE. If you use
notepad to open the project file [XXX].csproj, you can find the snippet
like this:

<ItemGroup>
   <Compile Include="Form5.cs">
     <SubType>Form</SubType>
   </Compile>
   <Compile Include="Form5.Designer.cs">
     <DependentUpon>Form5.cs</DependentUpon>
   </Compile>
   <Compile Include="Program.cs" />
   <Compile Include="Properties\AssemblyInfo.cs" />
   <EmbeddedResource Include="Form5.resx">
     <SubType>Designer</SubType>
     <DependentUpon>Form5.cs</DependentUpon>
   </EmbeddedResource>
   <EmbeddedResource Include="Properties\Resources.resx">
     <Generator>ResXFileCodeGenerator</Generator>
     <LastGenOutput>Resources.Designer.cs</LastGenOutput>
     <SubType>Designer</SubType>
   </EmbeddedResource>
   <Compile Include="Properties\Resources.Designer.cs">
     <AutoGen>True</AutoGen>
     <DependentUpon>Resources.resx</DependentUpon>
   </Compile>
   <None Include="Properties\Settings.settings">
     <Generator>SettingsSingleFileGenerator</Generator>
     <LastGenOutput>Settings.Designer.cs</LastGenOutput>
   </None>
   <Compile Include="Properties\Settings.Designer.cs">
     <AutoGen>True</AutoGen>
     <DependentUpon>Settings.settings</DependentUpon>
     <DesignTimeSharedInput>True</DesignTimeSharedInput>
   </Compile>
</ItemGroup>

These XML style attributes tell the VS IDE how to treat and open various
source *.cs files in the IDE designer. For example, <SubType>Form</SubType>
tells the IDE to launch form designer to view&edit the *.cs file. While
<DependentUpon>Form5.cs</DependentUpon> tells the IDE to place the current
*.cs item nested under "Form5.cs". So I think <DependentUpon> is what you
wanted.

Thanks.

Best regards,
Jeffrey Tan
Microsoft Online Community Support
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you can download free addin to visual studio from here:
http://www.kooriyoo.com/VSCommands/Default.aspx
and group items from VS IDE without need to add DependentUpon manually
to project file.