Comment vérifier si un tag xml existe ou pas

Bonjour,
Je dois réaliser un programme qui traite des documents XML.
Dans ce cadre, le professeur nous demande de tester si certains tag existe ou pas, ce qui engendrerai une erreur si pas. Je n'ai rien trouvé de concret la dessus sur le net :?.
voici un extrait de mon code:
Code:
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For Each node In element
                For Each nodeChild In node.ChildNodes
                    Select Case nodeChild.LocalName
                        Case "DOCUMENTID"
                            documentID = nodeChild.InnerText
                        Case "INVOICENR"
                            invoiceNr = nodeChild.InnerText
                        Case "INTERNAL_REFERENCE"
                            internalRef = nodeChild.InnerText
                        Case "JOURNAL"
                            journal = nodeChild.InnerText
                        Case "CUSTOMERNUMBER"
                            customerNumber = nodeChild.InnerText
                        Case "INVOICEDATE"
                            invoiceDate = nodeChild.InnerText
                        Case "VATRATE"
                            vatRate = nodeChild.InnerText
                        Case "DUEPERIOD"
                            duePeriod = nodeChild.InnerText
                        Case "DUE_DATE"
                            dueDate = nodeChild.InnerText
                        Case "INVOICETYPE"
                            invoiceType = nodeChild.InnerText
                        Case "FINANCIAL_TYPE"
                            financialType = nodeChild.InnerText
                        Case "ATTENTIONOF"
                            attentionOf = nodeChild.InnerText
                        Case "COMPANY"
                            company = nodeChild.InnerText
                        Case "DEPARTMENT"
                            department = nodeChild.InnerText
                        Case "INVOICECURRENCY"
                            invoiceCurrency = nodeChild.InnerText
                        Case "EXCHANGE_RATE"
                            exchangeRate = nodeChild.InnerText
                        Case "INVOICEDESCRIPTION"
                            invoiceDescription = nodeChild.InnerText
                        Case "TOTALAMOUNT"
                            totalAmount = nodeChild.InnerText
                        Case "TOTALAMOUNTVAT"
                            totalAmountVat = nodeChild.InnerText
                        Case "VAT_NO"
                            vatNo = nodeChild.InnerText
                        Case "VAT_CODE"
                            vatCode = nodeChild.InnerText
                        Case "POSTINGDATE"
                            postingDate = nodeChild.InnerText
                        Case "GENERAL_LEDGER"
                            generalLedger = nodeChild.InnerText
                        Case "SOURCE"
                            source = nodeChild.InnerText
                        Case "INTERNAL_NO"
                            internalNo = nodeChild.InnerText
                        Case "COMPANYNR"
                            companyNr = nodeChild.InnerText
                        Case "INVOICE_NO_SUPPLIER"
                            invoiceNoSupplier = nodeChild.InnerText
                        Case "CREATE_TIMESTAMP"
                            createTimestamp = nodeChild.InnerText
                    End Select
                Next
            Next
Merci d'avance :D