C# read data from XML File

Hello Experts,

i try to read data from XML File but i always get an Error: Data at the root level is invalid. Line 1, position 1.

The Error must be there:

XmlDocument doc = new XmlDocument();
doc.LoadXml(xml);
doc.PreserveWhitespace = false;
XmlNode root = doc.DocumentElement;

Can someone help me? 

Kind Regards
Léon