Search XDocument using LINQ without knowing the namespace

As Adam precises in the comment, XName are convertible to a string, but that string requires the namespace when there is one. That's why the comparison of .Name to a string fails, or why you can't pass "Person" as a parameter to the XLinq Method to filter on their name.
XName consists of a prefix (the Namespace) and a LocalName. The local name is what you want to query on if you are ignoring namespaces.
Thank you Adam :)

You can't put the Name of the node as a parameter of the .Descendants() method, but you can query that way :

var doc= XElement.Parse(
@"<s:Envelope xmlns:s=""http://schemas.xmlsoap.org/soap/envelope/"">
<s:Body xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xmlns:xsd=""http://www.w3.org/2001/XMLSchema"">
  <Request xmlns=""http://CompanyName.AppName.Service.ContractA"">
    <Person>
        <CreditCardNumber>83838</CreditCardNumber>
        <FirstName>Tom</FirstName>
        <LastName>Jackson</LastName>
    </Person>
    <Person>
        <CreditCardNumber>789875</CreditCardNumber>
        <FirstName>Chris</FirstName>
        <LastName>Smith</LastName>
    </Person>
   </Request>
   </s:Body>
</s:Envelope>");

EDIT : bad copy/past from my test :)

var persons = from p in doc.Descendants()
              where p.Name.LocalName == "Person"
              select p;

foreach (var p in persons)
{
    Console.WriteLine(p);
}

That works for me...

You could take the namespace from the root-element:

XDocument xDocument = XDocument.Load(@"C:\temp\Packet.xml");
var ns = xDocument.Root.Name.Namespace;

Now you can get all desired elements easily using the plus-operator:

root.Elements(ns + "CreditCardNumber")