string - C# str.Replace only replaces first character -

this code, know isn't optimal, works me.

 private void textbox_title1_textchanged(object sender, eventargs e)     {         string title1 = textbox_title1.text;         string currentdirectory = directory.getcurrentdirectory();         datetime folderdate = datetime.today;         string workingfolder = folderdate.tostring("ddmmmyy");         string mailingdir = ("mailingdir\\");         string indexpage = (mailingdir + "\\" + workingfolder + "\\" + "index.html");         string appdir = path.getdirectoryname(application.executablepath);         string title1open = path.combine(appdir, mailingdir, workingfolder, "index.html");         string str = file.readalltext(title1open, encoding.utf8);          str = str.replace("[title1]", title1);         file.writealltext(title1open, str);     } 

however, saves first character entered textbox_title1