Find out the default namespace URI from an XML document in C#

var doc = XDocument.Parse(myXml);
XNamespace ns = doc.Root.GetDefaultNamespace();

If you are using XmlDocument, you can get the default namespace by checking NamespaceURI of the root element:

var document = new XmlDocument();
document.LoadXml("<root xmlns='http://java.sun.com/xml/ns/j2ee'></root>");
var defaultNamespace = document.DocumentElement.NamespaceURI;
Assert.IsTrue(defaultNamespace == "http://java.sun.com/xml/ns/j2ee");