c#用线程遍历磁盘下的文件

我是一个初学者，不知道错在哪listbox上不显示信息。请高手们看看        
public class getFf
        {
            public   DirectoryInfo dds;
            public getFf(DirectoryInfo fs)
            {
                this.dds = fs;
            }
            public void  ThreadF()
            {
                if (dds != null)
                {
                    try
                    {
                        filePathShow fps = new filePathShow();
                        fps.WalkDirectoryTree(dds);                    }
                    catch (Exception)
                    {
                        throw;
                    }
                }
            }
        }
        private void btnStart_Click(object sender, EventArgs e)
        {
            //Control.CheckForIllegalCrossThreadCalls = false;
                        DriveInfo[] di = DriveInfo.GetDrives();
            foreach (DriveInfo dri in di)
            {
                if (dri.IsReady)
                {
                    if (dri.Name != @"C:\")
                    {
                        try
                        {
                            DirectoryInfo dInfo = dri.RootDirectory;
                            WalkDirectoryTree(dInfo);
                            getFf gf = new getFf(dInfo);
                            Thread thread = new Thread(new ThreadStart(gf.ThreadF));                            thread.Start();
                            //WalkDirectoryTree(dba);
                        }
                        catch (Exception)
                        {
                            throw;
                        }
                    }
                }
            }
         }            public void WalkDirectoryTree(DirectoryInfo root)
            {                FileInfo[] files = null;
                DirectoryInfo[] subDirs = null;                try
                {
                    files = root.GetFiles("*.*");
                }
                catch (UnauthorizedAccessException e)
                {                }                if (files != null)
                {
                    foreach (System.IO.FileInfo fi in files)
                    {
                        //在listbox上显示
                        lbxShow.Items.Add(fi.FullName);
                        lbxShow.Items.Add(fi.Name);
                    }                    subDirs = root.GetDirectories();                    foreach (System.IO.DirectoryInfo dirInfo in subDirs)
                    {
                        WalkDirectoryTree(dirInfo);
                    }
                }
            }
        }