<?xml version="1.0" encoding="utf-8"?>
<ns0:Entity xmlns:ns0="http://schemas.mycompany.com/1.0">
    <ns0:Data>
        <Address_Id xmlns:ns0="http://schemas.mycompany.com/1.0">7</Address_Id>
        <Customer_Id xmlns:ns0="http://schemas.mycompany.com/1.0">67</Customer_Id>
        <CustomerName xmlns:ns0="http://schemas.mycompany.com/1.0">My Customer 1</CustomerName>
    </ns0:Data>
</ns0:Entity>

private static string GetValue_Attempt1(XPathDocument xPathDocument, string xpathExpression)
{
    var xpathNavigator = xPathDocument.CreateNavigator();
    var xpathNodeIterator = xpathNavigator.Select(xpathExpression);
    xpathNodeIterator.MoveNext();
    return xpathNodeIterator.Current.Value;
}

private static string GetValue_Attempt2(XPathDocument xPathDocument, string xpathExpression)
{
    var xpathNavigator = xPathDocument.CreateNavigator();
    var xpathNodeIterator = xpathNavigator.Select(xpathExpression);
    xpathNodeIterator.MoveNext();
    var nodesNavigator = xpathNodeIterator.Current;
    var nodesText = nodesNavigator.SelectDescendants(XPathNodeType.Text, false);
    nodesText.MoveNext();
    return nodesText.Current.Value;
}