How to iterate through each word, ms-word?

Ok, my main objective is to go through each word and check if word is underlined. If it is, I want to change the font size to an int x.

I have tried simply going through each character like so Edit: Updated code

   private void button1_Click(object sender, EventArgs e)
    {
        word.Application page = new word.Application();
        page.Visible = true;
        word.Document doc = null;
        foreach (string fi in listBox1.Items)
        {
            doc = page.Documents.Open(Application.StartupPath + "\\old\\" + fi);
            if (doc != null)
            {
                int start = 0;
                foreach (string text in doc.Range().Text.Split(" \r\n\t.".ToCharArray()))
                {
                    int x = doc.Range().Text.IndexOf(text, start);
                    if (doc.Range(x, text.Length - 1).Underline == word.WdUnderline.wdUnderlineSingle)
                        doc.Range(x, text.Length - 1).Font = new word.Font() { Name = "Times New Roman", Bold = 4, Size = 12 };
                    else
                        doc.Range(x, text.Length - 1).Font = new word.Font() { Name = "Times New Roman", Size = 8 };
                    start = x+text.Length;
                }
            }
        }
        //doc.Save();
        // doc.Close();
        // page.Quit();
    }

But, I get this error

Call was rejected by callee. (Exception from HRESULT: 0x80010001 (RPC_E_CALL_REJECTED))

I have no idea why it gives that

Your code could be improved upon heavily:

doc = page.Documents.Open(System.IO.Path.Combine(Application.StartupPath, "old", fi));
if (doc != null)
{
     word.Font RegularFont = new word.Font() { Name = "Times New Roman", Size = 12 };
     word.Font BigFont = new word.Font() { Name = "Times New Roman", Size = 8 };

     for (int x = 1; x <= doc.Words.Count; x++)
     {
          if (doc.Words[x].Underline != word.WdUnderline.wdUnderlineNone && doc.Words[x].Underline != word.WdUnderline.wdUnderlineDouble)
               doc.Words[x].Font = RegularFont;
           else
                doc.Words[x].Font = BigFont;
      }
}