public override string ToString()
{
StringBuilder result = new StringBuilder ();
string value;
foreach (string key in Keys){
result.Append (key);
value = this [key] as string;
if (value != null)
result.AppendFormat ("=\"{0}\"", value);
result.Append (' ');
}
if (result.Length > 0 && result [result.Length - 1] == ' ')
result.Length--;
return result.ToString ();
}