Loop through main folder and subfolders

I only manage to produce the below code for looping through all the XML files in a folder and write into a csv file.

XmlDocument xmlDoc = new XmlDocument();
string[] fileEntries = Directory.GetFiles(@"..\..\bin\Debug\commentsXML\");
foreach (string fileName in fileEntries)
{
    try
    {
        string docPath = fileName;
        //load the XML document from the specified file
        xmlDoc.Load(docPath);
        XmlNodeList nodeList = xmlDoc.GetElementsByTagName("item");
        using (FileStream fs = new FileStream(@"..\..\bin\Debug\" + "Comments.csv", FileMode.Append))
        using (StreamWriter sw = new StreamWriter(fs))
        {
            foreach (XmlNode node in nodeList)
            {
                //do the writing
            }
            sw.Flush();
            sw.Close();
            fs.Close();
        }
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message);
    }
}
MessageBox.Show("Done writing into comments.csv!");

The above code is for the folder/file structure of:

|--Main Folder

|---XML1

|---XML2

|---XML3

What if I have a new folder/file structure like below? Could anyone be kind enough to point me out how to write/modify the foreach loop?

Main Folder

• Subfolder1
  
  
  
  • XML1
  
  
  • XML2
  
  
  • XML3
  
  
  
  


• Subfolder2
  
  
  
  • XML1
  
  
  • XML2
  
  
  • XML3
  
  
  
  


• Subfolder3
  
  
  
  • XML1
  
  
  • XML2
  
  
  • XML3