How do I create a custom Visual Studio 2008 UI designer for a C# file?
For example, when you double click on a DataSet in the Solution Explorer, a UI screen appears that allows you to edit the DataSet, even though it is defined in XML/code (which you can right click and "View Code").
Usually this code is separated from user code in some way, either by region ("Windows Forms Designer Generated Code"), by codegen (".g.cs" for WPF XAML files), or some other means like partial classes.
For some hints on Visual Studio Extensibility, see "Visual Studio 2010 addin writing articles/tutorials?". The Visual Studio SDK may have the information you need.