<SendRequestRequest xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="urn://x-artefacts-smev-gov-ru/services/message-exchange/types/basic/1.1">
  <SenderProvidedRequestData Id="SIGNED_BY_CONSUMER">
    <MessageID xmlns="urn://x-artefacts-smev-gov-ru/services/message-exchange/types/1.1">Значение0</MessageID>
    <MessagePrimaryContent>
      <tns:Request xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:tns="urn://ru.gov.proc.erp.communication/2.0.5" xmlns:erp_types="urn://ru.gov.proc.erp.communication/types/2.0.5" xsi:schemaLocation="urn://ru.gov.proc.erp.communication/2.0.5 ru.gov.proc.erp.communication.smev.xsd">
        <erp_types:MessageCommon InfoModel="Значение1" PreviousInfoModel="0">
          <erp_types:ProsecutorAsk />
        </erp_types:MessageCommon>
      </tns:Request>
    </MessagePrimaryContent>
    <TestMessage xmlns="urn://x-artefacts-smev-gov-ru/services/message-exchange/types/1.1" />
  </SenderProvidedRequestData>
  <CallerInformationSystemSignature>
    <Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns="">
      <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:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
          </ds:Transforms>
          <ds:DigestMethod Algorithm="http://www.w3.org/2001/04/xmldsig-more#gostr3411" />
          <ds:DigestValue>Значение3</ds:DigestValue>
        </ds:Reference>
      </ds:SignedInfo>
      <ds:SignatureValue>Значение4</ds:SignatureValue>
      <KeyInfo>
        <X509Data>
          <X509Certificate>Значение5</X509Certificate>
        </X509Data>
      </KeyInfo>
    </Signature>
  </CallerInformationSystemSignature>
</SendRequestRequest>

var Contact = from xml2 in xdoc.Descendants("CallerInformationSystemSignature") select xml2;
foreach (var item in Contact)
{                 

}

foreach (var item in Contact2)
{

}

var Contact4 = from xml2 in xdoc.Elements("SenderProvidedRequestData").Elements("CallerInformationSystemSignature") select xml2;
foreach (var item in Contact4)
{

}