Записать в текстовый файл построчно.

 class IniRecord : IComparable<IniRecord>
    {
        public string Section { get; set; }
        public List<string> KeysVals { get; set; }
        //public List<string> Keys;
        //public List<string> Values;

        public IniRecord()
        {
            KeysVals = new List<string>();
        }

        public IniRecord(string sect)
        {
            Section = sect;
            KeysVals = new List<string>();
        }

        public int CompareTo(IniRecord compare_rec)
        {
            // A null value means that this object is greater. 
            if (compare_rec == null)
                return 1;

            else
                return this.Section.CompareTo(compare_rec.Section);
        }
    }

  List<IniRecord> records = ini_parser.LoadIniFileToStructure();
  records.Sort();

  File.WriteAllText(@"C:\DMP\test.ini", String.Empty);
  foreach (IniRecord rec in records)
  {
       File.AppendAllText(@"C:\DMP\test.ini", rec.Section + "\n\r");
       if (rec.KeysVals.Count > 0)
       {
           foreach (string str  in rec.KeysVals)
               File.AppendAllText(@"C:\DMP\test.ini", str + "\n\r");
       }
  }

using (FileStream fileStream = new FileStream(@"C:\DMP\test.ini", FileMode.Create, FileAccess.Write, FileShare.None))
using (StreamWriter w = new StreamWriter(fileStream))
{
	foreach (IniRecord rec in records)
	{
		w.WriteLine(rec.Section);
		if (rec.KeysVals.Count > 0)
		{
			foreach (string str  in rec.KeysVals)
			{
				w.WriteLine(str);
			}
		}
	}   
}

 foreach (string str  in rec.KeysVals)

StringBuilder.Append(string.Concat(string.Join(Environment.NewLine, rec.KeysVals), Environment.NewLine));

 foreach (string str  in rec.KeysVals)

StringBuilder.Append(string.Concat(string.Join(Environment.NewLine, rec.KeysVals), Environment.NewLine));

foreach (string str in rec.KeysVals)
    sw.WriteLine(str);

StringBuilder.Append(string.Concat(string.Join(Environment.NewLine, rec.KeysVals), Environment.NewLine));