从子目录返回特定的文件类型

通过暗示此调用，我得到了当前目录的名称。我在目录中有一个子目录列表。

Users\Xeon\Documents\Visual Studio2013\Projects\Consolesocket1\Consolesocket1\Data

调用之后，我得到一个名为folder的返回值，它是其中的子文件夹。这是我要完成的任务。现在我如何才能在此选中的每个子目录中检索具有特定文件类型的文件

private void button1_Click(object sender, EventArgs e)
 {
   FolderBrowserDialog folderBrowserDlg = new FolderBrowserDialog();            
   folderBrowserDlg.ShowNewFolderButton = true;            
   DialogResult dlgResult = folderBrowserDlg.ShowDialog();
   if (dlgResult.Equals(DialogResult.OK))
   {               
     textBox1.Text = folderBrowserDlg.SelectedPath;                
     Environment.SpecialFolder rootFolder = folderBrowserDlg.RootFolder;
   }
}
private void button2_Click(object sender, EventArgs e)
{
   string[] extensions = { ".xml", ".ddg" };
   string[] dizi = Directory.GetFiles(textBox1.Text, "*.*", SearchOption.AllDirectories)
                    .Where(f => extensions.Contains(new FileInfo(f).Extension.ToLower())).ToArray();

   string[] dizin = Directory.GetDirectories(textBox1.Text, "P*", SearchOption.TopDirectoryOnly);
   foreach (var i in dizin)
   {
     FileInfo f = new FileInfo(i);
     listBox1.Items.Add(f.Name);
   }
   string[] di = Directory.GetDirectories(textBox1.Text, "S*", SearchOption.TopDirectoryOnly);
   foreach (var z in di)
   {
      FileInfo f = new FileInfo(z);
      listBox1.Items.Add(f.Name);
   }
}