.NET Context Menu Handler

By Eric — — 3 minute read

I've been working on a context menu handler shell extension in C# lately. There are a few samples that people have produced on the web (for example, The Code Project, pek.com, and TheServerSide.net). I've found a better way of implementing IShellExtInit.Initialize than in the examples I've seen.

The point of IShellExtInit.Initialize is for the shell to let your handler know which files are selected so that you can display your context menu based on, believe it or not, context. Here is a typical implementation:

int IShellExtInit.Initialize (IntPtr pidlFolder, IntPtr lpdobj, uint hKeyProgID)
{
  try
  {
    // save the information about the selection
    m_dataObject = null;
    if (lpdobj != (IntPtr)0)
    {
      m_dataObject = (ShellLib.IDataObject)Marshal.GetObjectForIUnknown(lpdobj);
      FORMATETC fmt = new FORMATETC();
      fmt.cfFormat = CLIPFORMAT.CF_HDROP;
      fmt.ptd = 0;
      fmt.dwAspect = DVASPECT.DVASPECT_CONTENT;
      fmt.lindex = -1;
      fmt.tymed = TYMED.TYMED_HGLOBAL;
      STGMEDIUM medium = new STGMEDIUM();
      m_dataObject.GetData(ref fmt, ref medium);
      m_hDrop = medium.hGlobal;
    }
  }
  catch(Exception)
  {
  }
  return 0;
}

Then later on, to get the selected file:

// Get the file name to work with
StringBuilder sb = new StringBuilder(1024);
Helpers.DragQueryFile(m_hDrop, 0, sb, sb.Capacity + 1);

There's a fair amount of nasty interop going on here, which is sometimes inevitable for things like shell extensions. In this case, however, you can actually use the .NET Framework's DataObject class instead for much simpler code:

int IShellExtInit.Initialize (IntPtr pidlFolder, IntPtr lpdobj, uint hKeyProgID)
{
  DataObject dataObject = new DataObject(lpdobj);
  string[] selectedFiles = (string[])dataObject.GetData(DataFormats.FileDrop);
}