Call GetOpenFileName() from C#

I tried a sample code provided in Microsoft .NET on how to make a call to
Win32 GetOpenFileName. It crashed if I call the function, select a file and
close the dialog and repeat the process several times.. I got the following
error :

An unhandled exception of type 'System.NullReferenceException' occurred in
system.windows.forms.dll

Additional information: Object reference not set to an instance of an
object.

Below is the code :

  [ StructLayout( LayoutKind.Sequential, CharSet=CharSet.Auto )]
  public class OpenFileName
  {
   public int  structSize = 0;
   public IntPtr dlgOwner = IntPtr.Zero;
   public IntPtr instance = IntPtr.Zero;

   public String filter = null;
   public String customFilter = null;
   public int  maxCustFilter = 0;
   public int  filterIndex = 0;

   public String file = null;
   public int  maxFile = 0;

   public String fileTitle = null;
   public int  maxFileTitle = 0;

   public String initialDir = null;

   public String title = null;

   public int  flags = 0;
   public short fileOffset = 0;
   public short fileExtension = 0;

   public String defExt = null;

   public IntPtr custData = IntPtr.Zero;
   public IntPtr hook = IntPtr.Zero;

   public String templateName = null;

   public IntPtr reservedPtr = IntPtr.Zero;
   public int  reservedInt = 0;
   public int  flagsEx = 0;
  }

  public class LibWrap
  {
   [ DllImport( "Comdlg32.dll", CharSet=CharSet.Auto )]
   public static extern bool GetOpenFileNameW([ In, Out ] OpenFileName
ofn );
  }

  private void Test()
  {
   OpenFileName ofn = new OpenFileName();

   ofn.structSize = Marshal.SizeOf( ofn );
   ofn.filter = "*.xml" ;
   ofn.file = new String( new char[ 256 ]);
   ofn.maxFile = ofn.file.Length;
   ofn.fileTitle = new String( new char[ 64 ]);
   ofn.maxFileTitle = ofn.fileTitle.Length;
   ofn.initialDir = "C:\\";
   ofn.title = "Open file";
   ofn.defExt = "xml";
   ofn.flags = 0x00000100;
   ofn.file = "*.xml";
   ofn.dlgOwner = this.Handle;

   if( LibWrap.GetOpenFileNameW( ofn ))
   {
   }
  }

Any Idea ?