In the solution explorer you can see nested files:

Let's see how to do the same with the two files Class1.cs
and Class1.Custom.cs
. The trick is in the project file (csproj). You need to look for the Class1.Custom.cs
file in it and indicate which item is its parent.
<Compile Include="Class1.cs" />
<Compile Include="Class1.Custom.cs">
<DependentUpon>Class1.cs</DependentUpon>
</Compile>
And here's the result:

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