private static void InvokeCallbackCommand( OpenFileInteraction openFile, IReadOnlyList<IFile> files )
        {
            Contract.Requires( openFile != null );
            Contract.Requires( files != null );

            if ( files.Any() )
            {
                // set file and execute accept
                openFile.Files.ReplaceAll( files );
                openFile.ExecuteDefaultCommand();
            }
            else
            {
                // execute cancel
                openFile.ExecuteCancelCommand();
            }
        }