Add status icons over file icons in Explorer, like Dropbox or SVN in .NET

For anyone still interessted in this question:

Here is a codeproject link which describes the process in great detail.

It uses the SharpShell library, which can also be found on nuget.

Code, from the codeproject, looks something like that:

[ComVisible(true)]
public class ReadOnlyFileIconOverlayHandler : SharpIconOverlayHandler
{
  protected override int GetPriority()
  {
    //  The read only icon overlay is very low priority.
    return 90;
  }

  protected override bool CanShowOverlay(string path, FILE_ATTRIBUTE attributes)
  {
    try
    {
      //  Get the file attributes.
      var fileAttributes = new FileInfo(path);

      //  Return true if the file is read only, meaning we'll show the overlay.
      return fileAttributes.IsReadOnly;
    }
    catch (Exception) { return false; }
  }

  protected override System.Drawing.Icon GetOverlayIcon()
  {
    //  Return the read only icon.
    return Properties.Resources.ReadOnly;
  }
}

I've never play with this, but I think it's the right way.

Custom Folder

First make the folder a System Folder, then create a Desktop.ini file and apply the change inside.

[.ShellClassInfo]
[email protected],-12690
IconFile=%SystemRoot%\system32\SHELL32.dll
IconIndex=-238  

You should develop an overlay icon handler and register it into the system.
Here you can find a partially working example written in C#.
Some MSDN documentation here and here.