<p>In Visual Studio, two files are created when you create a new Windows Form in your solution (e.g. if you create MyForm.cs, MyForm.Designer.cs and MyForm.resx are also created). These second two files are displayed as a subtree in the Solution Explorer.</p>

<p><strong>Is there any way to add files to the sub-tree or group for a Windows Form class?</strong></p>

You need to edit the csproj directly. There is a DependentUpon tag that you have to add as a child tag of the file you want to place under MyForm.cs.
Example:
```
```
<Compile Include="MyForm.MyCoolSubFile.cs">
  <DependentUpon>MyForm.cs</DependentUpon>
</Compile>
```
```