public string AtributeOuterXml(string name, string value) {
if(encoder == null) Init();
buffer .Length = 0; // clean buffer
buffer .Append(name);
buffer .Append('=');
buffer .Append(QuoteChar);
encoder.StartAttribute(/*save:*/false);
encoder.Write(value);
encoder.EndAttribute();
buffer .Append(QuoteChar);
return buffer.ToString();
}