var reqNodes = from el in xdoc.Root.Descendants()
                                   where el.Name.LocalName == "Signature"
                                   select el;

                    foreach (var reqNode in reqNodes)
                    {
                        
                        XNamespace xns = reqNode.Name.Namespace;
                        //Queries making use of namespace:
                        var SignedInfo = (from el in reqNode.Elements("SignedInfo")
                                          select el).ToList();
                        foreach (var item in SignedInfo)
                        {
#### сюда компилятор не попадает почему-то...
                        }

                        var SignatureValue = (from el in reqNode.Elements(xns + "SignatureValue")
                                              select el).First().Value;

                        var KeyInfo = (from el in reqNode.Elements("KeyInfo")
                                       select el).ToList();
                    }

<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
  <ds:SignedInfo>
    <ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
    <ds:SignatureMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#gostr34102001-gostr3411" />
    <ds:Reference URI="#SIGNED_BY_CONSUMER">
      <ds:Transforms>
        <ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
        <ds:Transform Algorithm="urn://smev-gov-ru/xmldsig/transform" />
      </ds:Transforms>
      <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#gostr3411" />
      <ds:DigestValue>xxxxxxxxxxxxxxxxxxxxxxx</ds:DigestValue>
    </ds:Reference>
  </ds:SignedInfo>
  <ds:SignatureValue>gggggggggggggggggggggg</ds:SignatureValue>
  <ds:KeyInfo>
    <ds:X509Data>
      <ds:X509Certificate>zzzzzzzzzzzzzzzzzzzzzzz</ds:X509Certificate>
    </ds:X509Data>
  </ds:KeyInfo>
</ds:Signature>