OpenFileDialog

Contents of OpenFileDialog.cs:

using System;
using System.Windows.Forms;

class MainClass
{
    public static void Main(string[] args)
    {
        OpenFileDialog o = new OpenFileDialog();
    }
}

Compiled by:
gmcs /reference:System.Windows.Forms.dll OpenFileDialog.cs

When running:
mono OpenFileDialog.exe

Mono System.Windows.Forms Assembly [$auto_build_revision$]
Gtk colorscheme read

Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object
  at System.Windows.Forms.MimeIconEngine.GetIconIndexForMimeType (System.String mime_type) [0x00000]
  at System.Windows.Forms.UnixFileSystem..ctor () [0x00000]
  at System.Windows.Forms.MWFVFS..ctor () [0x00000]
  at System.Windows.Forms.FileDialog..ctor () [0x00000]
  at System.Windows.Forms.OpenFileDialog..ctor () [0x00000]
  at (wrapper remoting-invoke-with-check) System.Windows.Forms.OpenFileDialog:.ctor ()
  at MainClass.Main (System.String[] args) [0x00000]