c# XPathSelectElement return null even if the namespace is specified

Question

I have this xml document in a file:

<samlp:AuthnRequest xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="_73c4b43a-d201-4990-b656-e6bab19e1c09" Version="2.0" IssueInstant="2021-12-14T08:09:39.816485Z" Destination="https://localhost/idp/sso/post" ForceAuthn="true" AssertionConsumerServiceIndex="0" AssertionConsumerServiceURL="https://localhost:5002/signin-spid" AttributeConsumingServiceIndex="0" xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" >
<saml:Issuer NameQualifier="https://localhost:5002" Format="urn:oasis:names:tc:SAML:2.0:nameid-format:entity" >https://localhost:5002</saml:Issuer>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
        <CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
        <SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#rsa-sha256" />
        <Reference URI="#_73c4b43a-d201-4990-b656-e6bab19e1c09">
            <Transforms>
                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
            </Transforms>
            <DigestMethod Algorithm="http://www.w3.org/2001/04/xmlenc#sha256" />
            <DigestValue>.........DigestValue...........</DigestValue>
        </Reference>
    </SignedInfo>
    <SignatureValue>..............Signature................</SignatureValue>
    <KeyInfo>
        <X509Data>
            <X509Certificate>...........Certificate.............</X509Certificate>
        </X509Data>
    </KeyInfo>
</Signature>
<samlp:NameIDPolicy Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient" />
<saml:Conditions NotBefore="2021-12-14T07:59:39.816485Z" NotOnOrAfter="2021-12-14T08:19:39.816485Z" />
</samlp:AuthnRequest>

I just trying to get the SignatureValue tag value and X509Certificate tag value with this code:

var xDocument = XDocument.Load("Request.xml");
var reader = xDocument.CreateReader();

var namespaceManager = new XmlNamespaceManager(reader.NameTable);
namespaceManager.AddNamespace("", "http://www.w3.org/2000/09/xmldsig#");
var signature = xDocument.XPathSelectElement("/Signature/SignatureValue", namespaceManager);
var x509Certificate = xDocument.XPathSelectElement("/Signature/KeyInfo/X509Data/X509Certificate", namespaceManager);

but Signature and x509Certificate Elements are always null. I also tried to use the method directly on the Root object but it doesn't work the same. What am I doing wrong?
Thanks

Do you have to use XmlReader and XPathSelectElement here? There are simpler ways of getting the right element using LINQ to XML. Note that you're asking for /Signature, but the root element is AuthnRequest, then Signature inside that. Perhaps that's actually the problem? — Jon Skeet
– Jon Skeet, Commented Dec 14, 2021 at 11:41
I just used the / why it should go down one node under AuthnRequest, but anyway even specifying AuthnRequest / Signature doesn't work — pampua84
– pampua84, Commented Dec 14, 2021 at 12:46
Okay, and what about my question about whether you have to use XPathSelectElement? If you're happy with using the Element method then it's trivial. — Jon Skeet
– Jon Skeet, Commented Dec 14, 2021 at 12:47
I'm using other methods now, but I was really curious to understand why it didn't work — pampua84
– pampua84, Commented Dec 14, 2021 at 12:52
Okay, if your question is specific to using NamespaceManager, please fix the example to include /AuthnRequest. But I suspect the note in learn.microsoft.com/en-us/dotnet/api/… is relevant here - you should be specifying a non-empty prefix, I suspect. — Jon Skeet
– Jon Skeet, Commented Dec 14, 2021 at 13:04

William Walseth · Accepted Answer · 2021-12-14 14:23:26Z

2

Here's a quick one. You have to give a prefix to the empty default namespace. It can be anything really, in this example I called it "root".

using System.Xml;

NameTable nt = new NameTable();
XmlNamespaceManager nsmgr = new XmlNamespaceManager(nt);
nsmgr.AddNamespace("root", "http://www.w3.org/2000/09/xmldsig#");

XmlDocument xml = new XmlDocument();
xml.Load("c:\\temp\\test.xml");
XmlNode ndSignature = xml.SelectSingleNode("//root:Signature/root:SignatureValue", nsmgr);
XmlNode ndCertificate = xml.SelectSingleNode("//root:Signature/root:KeyInfo/root:X509Data/root:X509Certificate", nsmgr);

Console.WriteLine(ndSignature.InnerText);
Console.WriteLine(ndCertificate.InnerText);

answered Dec 14, 2021 at 14:23

William Walseth

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2 Comments

pampua84 Over a year ago

Thanks, the problem was just to indicate the prefix and put it before the name of each child. It works perfectly with this code: ``` xmlNamespaceManager.AddNamespace("sig", "w3.org/2000/09/xmldsig#"); var signatureValue = xDocument.XPathSelectElement("//sig:Signature/sig:SignatureValue", xmlNamespaceManager); ```

William Walseth Over a year ago

Names spaces create more problems than they solve. Somehow, we can all use hierarchical file system without worry of 2 files with the same name. We just know to put them in different folders (=parent elements). I've never understood why XML elements are any different.

Yitzhak Khabinsky · Accepted Answer · 2021-12-14 14:37:08Z

2

It is better to use LINQ to XML API.

It is available in the .Net Framework since 2007.

c#

void Main()
{
    const string filename = @"e:\Temp\pampua84.xml";

    XDocument xdoc = XDocument.Load(filename);
    XNamespace ns = "http://www.w3.org/2000/09/xmldsig#";
    

    string SignatureValue = xdoc.Descendants(ns + "SignatureValue")
        .FirstOrDefault()?.Value;

    string X509Certificate = xdoc.Descendants(ns + "X509Certificate")
        .FirstOrDefault()?.Value;

    Console.WriteLine("SignatureValue='{0}'", SignatureValue);
    Console.WriteLine("X509Certificate='{0}'", X509Certificate);
}

answered Dec 14, 2021 at 14:37

Yitzhak Khabinsky

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