Xmldocument encoding question.

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I have some code like this:

public static string SerializObject( object obj)
{
string xmlString = string.Empty;
using (MemoryStream msReq = new MemoryStream())
{
using (XmlTextWriter wsReq = new XmlTextWriter(m sReq, UTF8))
{
XmlSerializer xsR = new XmlSerializer(o bj.GetType());
xsR.Serialize(w sReq, obj);
xmlString = new
UTF8Encoding(). GetString(msReq .GetBuffer());
}
}
return xmlString;
}

Object obj;
string str = SerializObject( obj) --- Using UTF 8 to do the encoding.

Xmldocument doc = new Xmldocument();
doc.LoadXml(str );
string objStr = doc.InnerText;

Because I found in MSDN, it said all the string are represented as UTF-16.

Here I have a question is:
1. the string str is UTF-16 or UTF -8 ?
2. the string objStr is UTF-16 or UTF -8 ?

Thanks for your help in advance.